Despite Thunderbolt, iPad to Mac communication is still a mess

Posted:
in iPad edited April 15
The latest iPad Pros have Thunderbolt ports, so you'd expect improved performance when connecting to a Thunderbolt-equipped MacBook Pro. Instead, it's a buggy mess.

Connecting an iPad Pro to a MacBook Pro over Thunderbolt is a buggy experience
Connecting an iPad Pro to a MacBook Pro over Thunderbolt is a buggy experience


Users report that their iPad Pro with Thunderbolt would not connect to the latest MacBook Pros running Apple Silicon. So, AppleInsider tested and troubleshot the issue on a fifth-generation 12.9-inch iPad Pro and 14-inch MacBook Pro with an M1 Pro processor.

Posts on Apple's support website suggest that the iPad Pro wouldn't connect to a MacBook Pro over Thunderbolt, period. Others state that they had more luck when using Apple-made cables, but there were still issues with maintaining a connection.

We tested the connection issue first using Apple's Thunderbolt 4 cable included with the Studio Display and a Satechi Thunderbolt 4 cable that was newly purchased. When either cable was used, the iPad Pro would flash in and out of Finder as if the cable was shorted or broken somehow.

So, the MacBook Pro was at least acknowledging the iPad Pro was being connected, but it wouldn't stay mounted in the Finder long enough for any real interactions. We were able to routinely reproduce this using any Thunderbolt 4 cable with the iPad Pro, but non-Thunderbolt cables worked fine. The iPad mini 6 with USB-C connected normally when using any cable, including the Thunderbolt cables, since it used the USB 3.1 standard.

Users should expect normal behavior when connecting Thunderbolt products together
Users should expect normal behavior when connecting Thunderbolt products together


We bypassed this bug by repeatedly removing and inserting an assortment of Thunderbolt cables from the MacBook Pro's Thunderbolt port. Eventually, the connection remained stable with any one of a variety of cables, and files could be transferred and synced as usual.

However, this isn't a proper fix since it must be repeated each time you restart the Mac and want to reconnect the iPad.

While we weren't expecting much from them, we contacted Apple Support about this bug. The support representative took us through the usual troubleshooting steps as was expected, and they couldn't confirm if this was a known issue.

The issue persisted after restarting the Mac, reinstalling macOS, and performing a safe boot.

There are three Thunderbolt 4 ports in the 14-inch MacBook Pro
There are three Thunderbolt 4 ports in the 14-inch MacBook Pro


While there isn't a clear answer as to why this is the way it is, and the type of Thunderbolt cable be they active or passive doesn't seem to matter, it appears that macOS on Apple Silicon isn't expecting to interface with an iPad over Thunderbolt, despite a Thunderbolt iPad being around for over a year.

More on other Mac to iPad connectivity problems in a bit.

Another iPad Thunderbolt problem

While this isn't a related bug, it is certainly a problem that needs addressing while discussing syncing devices with a Mac. When AppleInsider was investigating the connection issues, we discovered a whole new problem relating to transfer speeds.

There isn't any clear reason why connecting an iPad to a Mac via Thunderbolt is buggy
There isn't any clear reason why connecting an iPad to a Mac via Thunderbolt is buggy


Some users in the support forums suggested that only Apple cables could be used to make data transfers, while others indicated that, at the least, they were superior. After getting the iPad Pro to remain connected to the MacBook Pro, we transferred a 5.9 GB file using different Thunderbolt cables to test that declaration.

The Apple Thunderbolt 4 cable could transfer a file at approximately 70MB/s, and any other branded cable had a speed of about 32MB/s. These speeds were verified via iStat Menus and manually timing the file transfer using a stopwatch.

These are still not blistering speeds. For reference, USB 2.0 is limited to about 35 megabytes per second in the real world. USB 3.0 can push around 350 megabytes per second, with 10 Gigabit USB 3.1 and 3.2 hitting 700 megabytes per second.

Thunderbolt 3 and Thunderbolt 4 can hit in excess of 2.8 gigabytes per second.

When using the same cables on an Intel Mac mini, a totally different behavior was observed, which means this may be an issue related to Apple Silicon. The iPad Pro could connect without issue to the Intel Mac mini, and it had a 200MB/s data transfer rate regardless of the cable being used.

The data speeds aren't ideal in either situation since Thunderbolt is rated in gigabytes, not megabytes. However, this may be a problem associated with how Apple handles file transfer to iPad and iPhone since it used a proprietary protocol through iTunes. It uses essentially the same protocol across a cable to sync data through the Finder from compatible sources like the Music, TV, Photos, and Books apps.

Connecting any iPad, even ones with USB-C, will result in slow data transfer speeds
Connecting any iPad, even ones with USB-C, will result in slow data transfer speeds


Prior to macOS Catalina, all syncing and iOS or iPadOS device connections were handled by iTunes and its internal processes. After Apple broke out the Music app and TV app into their own products, sync management was moved to Finder and new frameworks were put in use to manage this.

One active daemon called AMPDevicesAgent can be seen in Task Manager any time an iPhone or iPad is connected to a Mac. This is the syncing framework and has been known to get aggressive with its CPU use during sync. Some users have reported 100% CPU use just by plugging in a device.

This proprietary syncing protocol is probably behind most of the issues we're observing on both Intel and M-series Macs. We're not sure specifically why at the moment, but the likeliest scenarios are that the protocol has a maximum expected transfer rate artificially limiting device connection speeds.

However, it isn't clear if it is responsible for the poor Thunderbolt connection, or if that's a driver problem within Apple's Thunderbolt controllers themselves.

How this affects users

This bug isn't exactly a deal-breaker for most iPad users. In order to encounter these bugs you'd need to own both an iPad and a Mac and want to connect them for syncing or SideCar. The Thunderbolt connection issue is even more niche since you'd need the latest iPad Pro and a Mac running Apple Silicon.

Apple developers should be testing every use case, especially ones Apple advertises
Apple developers should be testing every use case, especially ones Apple advertises


Those who use their iPad Pro as a primary computer will never encounter this issue either since it is likely they'd never have to connect it to a Mac. So, it is probable that this use case is so niche that even Apple didn't run into it during testing.

After all, for every Mac sale, there are about 25 iPhone sales. And, the ratio is about the same for iPad to iPhone sales as well. This has led to a fragmented user base, with most iPhone-only, some Mac-only, some iPad-only, and a small fraction of the total user base being Mac plus iPad owners.

And as of late, ever since it removed the ability to sync apps from iTunes to the iPad over Lightning, syncing iPads over a cable is less of a priority for Apple, given the company's shift to iCloud backups and wireless syncing. Wired SideCar uses a different protocol than file transfers, but we're still surprised that Apple didn't do something about AMPDevicesAgent while it was in the neighborhood.

Either work on AMPDevicesAgent hasn't been a priority, which is why it still exists a year after releasing Thunderbolt-equipped iPad Pros, or Apple engineers simply missed it. Either situation isn't ideal since it shows a lack of concern for edge cases.

Apple hasn't made any official statement or acknowledged the bug in any way. We had hoped that contacting Apple Support and climbing up the chain to engineering specialists would provide some kind of positive result. Instead were met with standard scripted responses that resulted in asking us if we'd like to wipe our Mac to factory settings.

For now, users who want to connect their iPad Pro to an Apple Silicon Mac should stick to non-Thunderbolt USB-C cables and expect slow data speeds that aren't any better than early Lightning delivered.

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Comments

  • Reply 1 of 31
    geekmeegeekmee Posts: 574member
    I can still put up with this…
    As long as they keep their priorities focused on changing the enterprise computer industry standards with the M1.
    edited April 15 watto_cobra
  • Reply 2 of 31
    killroykillroy Posts: 214member
    While we are talking about M1 bugs. Anyone try to play a Blu-RAY image larger than 13gigs. It seems that the UDF driver is broken.
    It will send a M1 into a kernel panic and reboot. Intel Macs play it ok.
    OctoMonkeydarkvaderdewmeFileMakerFellerwatto_cobra
  • Reply 3 of 31
    I just plugged my iPad Pro into my Mac Studio and the Mac recognized it in Finder, but it is nowhere to be found in Photos. This reminds me of how I had problems syncing the iPhone 12 Pro Max and iPhone 13 Pro Max on an M1 Mac Mini. Apple needs to get its act together and improve the quality of its products. These issues are not excusable. Now I've got the same problem as before Photos doesn't see my iOS device except this time instead of my phone it is my iPad Pro. 
    edited April 15 elijahg
  • Reply 4 of 31
    entropysentropys Posts: 3,509member
    Thunderbolt every port is an issue too.  It makes things more expensive than it needs to be.
  • Reply 5 of 31
    Have a 16” MBP M1 and  the latest 12.9” iPad that I use for Sidecar when I’m away and it has been a disaster… the iPad unmounts or freezes every time, usually after 10 mins or so.  Happens when wired or wireless. I bought the iPad specifically for Sidecar. What a bummer.  Hope a software update fixes it.  
    watto_cobra
  • Reply 6 of 31
    With an M1 iPad Pro (12.9-inch) (5th generation) iPad Pro (12.9-inch) (5th generation) and an Intel iMac, using the Thunderbolt cable is consistently flaky (with regular failures) just trying to sync or update iPadOS.  I have to downgrade to the (theoretically slower) USB-C cable to be confident of everything working properly.
    watto_cobra
  • Reply 7 of 31
    Seems to be a firmware bug. However I've noticed that if I use a thunderbolt 4 dock with the m1 max as the host, connecting the iPad pro to one of the dock's downstream tb4 ports with a tb4 cable works. Direct Mac to iPad via a tb4 cable doesn't work, but a tb4 dock in the middle works. 

    Curiously, the iPad pro doesn't connect in thunderbolt mode, it connects via USB at 10 gbps.  So for some unknown reason whenever the iPad interfaces with the Mac via a tb3 or tb4 cable, a thunderbolt connection is not established, even though both ends have a thunderbolt controller. Apple needs to adress this. 

    Sidecar via WiFi works okay sometimes but sometimes it stutters or freezes. Usually restarting the connection works but sometimes I need to restart the iPad. Sidecar via USB works flawlessly. The only issue is YouTube tv displaying a black image in safari if sidecar is connected. But if I use opera or chrome(with hardware acceleration disabled) YouTube tv works well with sidecar connected. 
    edited April 16 FileMakerFellerAlex1N
  • Reply 8 of 31
    Maybe there is a very simple situation here. Perhaps the iPad Pro has its Thunderbolt set in host mode, and the M1 desktop/laptop has its Thunderbolt set in host mode, and the two hosts are trying to talk to each other and grapple for supremacy in the hierarchy, and the Finder dropouts are the result of one end unsuccessfully being deprecated to child, the connection resets, and after reset, both try to be adult hosts again. Repeat. 
    Fidonet127watto_cobra
  • Reply 9 of 31
    elijahgelijahg Posts: 2,614member
    XavierZed said:
    Maybe there is a very simple situation here. Perhaps the iPad Pro has its Thunderbolt set in host mode, and the M1 desktop/laptop has its Thunderbolt set in host mode, and the two hosts are trying to talk to each other and grapple for supremacy in the hierarchy, and the Finder dropouts are the result of one end unsuccessfully being deprecated to child, the connection resets, and after reset, both try to be adult hosts again. Repeat. 
    Almost certainly, though USB-C has a similar hierarchy and it works for that. The iPad is a tablet so should be lower down the “should I be a host” chain than a laptop, which is lower than a desktop. 

    Apple needs to pull their finger out and fix these sorts of bugs. They are way too common. Safari’s UI on iOS regularly breaks for me, the address bar floats halfway up the screen or disappears entirely. 

    Other companies pull off mostly bug-free software and are 10% the size of Apple with 20x the number of products, and don’t charge the Apple premium. There is no excuse, Cook needs to stop cutting the QA budget. 
  • Reply 10 of 31
    I didn't know people still connecting iPad to Mac using cable. I use AirDrop all the time between my Mac, iPad and iPhone. For Photos, iCloud automatically syncs across all devices including my Mac. Less headache this way.
    watto_cobra
  • Reply 11 of 31
    Big Hint: the M1 iPad Pro comes with a USB-C cable for connecting to Macs. 
    killroywatto_cobra
  • Reply 12 of 31
    Big Hint: the M1 iPad Pro comes with a USB-C cable for connecting to Macs. 
    Yes it does but the included cable does not work for connecting to an external drive. Using the supplied cable did not work. I had to use the original cable that came with the external drive, but hey maybe I didn’t plug it in right. 
  • Reply 13 of 31
    Skeptical said:
    Big Hint: the M1 iPad Pro comes with a USB-C cable for connecting to Macs. 
    Yes it does but the included cable does not work for connecting to an external drive. Using the supplied cable did not work. I had to use the original cable that came with the external drive, but hey maybe I didn’t plug it in right. 
    It definitely works with connecting to drives with a USB-C port. YouTube is full of videos of people connecting to USB-C portable SSDs etc. 
    watto_cobra
  • Reply 14 of 31
    nhughesnhughes Posts: 770editor
    Big Hint: the M1 iPad Pro comes with a USB-C cable for connecting to Macs. 
    It is a cheap USB-C cable that operates at USB 2.0 speeds, which significantly undercuts the capabilities of the Thunderbolt ports on both the Mac and the iPad Pro. It’s fine for charging, but is a poor data transfer cable. 
    killroymuthuk_vanalingamFileMakerFellerwatto_cobra
  • Reply 15 of 31
    do I understand correctly that we have a family of cables that look identical to the untrained eye, but have different functionality - and that buying the latest/greatest most expensive version doesn't guarantee backward compatibility? 

    Apologies for my naivety. I thought I understood this, and that if I bought a 100W capable TB4 cable it could do anything/everything. 
    FileMakerFellerwatto_cobra
  • Reply 16 of 31
    I had not reflected over being super-unique when it comes to upgrading both my iPad and my Mac to stay up to date with Apple software enhancements, as well as just being both a frequent Mac- and iPad user. My phone is a OnePlus 9 Android phone, which probably corners me into the 0.5 % group of users, lol.
  • Reply 17 of 31
    crowleycrowley Posts: 10,256member
    nhughes said:
    Big Hint: the M1 iPad Pro comes with a USB-C cable for connecting to Macs. 
    It is a cheap USB-C cable that operates at USB 2.0 speeds, which significantly undercuts the capabilities of the Thunderbolt ports on both the Mac and the iPad Pro. It’s fine for charging, but is a poor data transfer cable. 
    Really?  That's a pretty poor show from Apple.
    nhughesFileMakerFeller
  • Reply 18 of 31
    nhughesnhughes Posts: 770editor
    crowley said:
    nhughes said:
    Big Hint: the M1 iPad Pro comes with a USB-C cable for connecting to Macs. 
    It is a cheap USB-C cable that operates at USB 2.0 speeds, which significantly undercuts the capabilities of the Thunderbolt ports on both the Mac and the iPad Pro. It’s fine for charging, but is a poor data transfer cable. 
    Really?  That's a pretty poor show from Apple.
    Yeah, it’s unfortunate to say the least. They should ship a USB 3 cable with all pro devices, but they do not ship it with any of them, Macs included. See here:

    https://support.apple.com/en-us/HT208368

    The Apple USB-C Charge Cable is longer (2m) and also supports charging, but data-transfer speed is limited to 480Mbps (USB 2.0) and it doesn't support video. 
    muthuk_vanalingamFileMakerFeller
  • Reply 19 of 31
    Big Hint: the M1 iPad Pro comes with a USB-C cable for connecting to Macs. 
    I have M1 iPad Pro. The cable is meant for charging, not data transfer. Not all Thunderbolt/USB-C cables are created equal. I also use it to connect to MacBook Pro for Sidecar because MacOS recommends it. And it works fine as Sidecar.
    edited April 17 watto_cobra
  • Reply 20 of 31
    It's a mess with Lightning cables as well.

    Backupping my 8th gen iPad to my iMac, is quite a hard task. I have and use only original Apple cables, but every single one desconnects from the Mac doing an annoying looping chime, leading to errors and try-and-retry process — change USB port, unplug and pulg the cable, etc.

    And when it finally go fine, data transfer is sloooooooooooooooooooow!

    A waste of time, for every single time.

    And no, I don't want to raise my iCloud+ subscription from 50 to 200 GB in order to do online backups.

    I just want to do it locally, and it's always a mess.

    Never happened with the old 30-pin cable.
    FileMakerFeller
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