Users report install, crashing issues with latest MacBook Air EFI update
It appears Apple's most recent EFI update for mid-2011 Mac Book Air models contains a bug that in some cases fails to install, or more seriously creates system crashes.

Apple pushed out the MacBook Air EFI update 2.9 last week to address sleep/wake issues seen on mid-2011 hardware running OS X 10.9.2 and later.
On Tuesday, users began reporting the update as flawed on Apple's Support Communities forum, citing a number of problems including installation bugs and system lockups. The exact cause of the problem is unknown, though a corrupted file may be to blame as users with identical hardware are seeing varied negative effects.
One user with a "bricked" Air took their machine to an Apple Store Genius Bar, where the technician performed a "soft" SMC reset to successfully expunge the tainted EFI update and bring the computer back to life. The issue was then reported to Apple's engineering department, though the company has yet to remove the EFI update page.
The thread, which now has more than 850 views and 42 replies, contains a number of similar stories, though the number of MacBook Air owners affected remains unknown.
Apple's MacBook Air update was meant to fix an issue that causes the laptop to take longer than expected to wake from sleep. Also addressed in the software is a problem that could cause the computer's fans to run at full speed after waking from sleep.

Apple pushed out the MacBook Air EFI update 2.9 last week to address sleep/wake issues seen on mid-2011 hardware running OS X 10.9.2 and later.
On Tuesday, users began reporting the update as flawed on Apple's Support Communities forum, citing a number of problems including installation bugs and system lockups. The exact cause of the problem is unknown, though a corrupted file may be to blame as users with identical hardware are seeing varied negative effects.
One user with a "bricked" Air took their machine to an Apple Store Genius Bar, where the technician performed a "soft" SMC reset to successfully expunge the tainted EFI update and bring the computer back to life. The issue was then reported to Apple's engineering department, though the company has yet to remove the EFI update page.
The thread, which now has more than 850 views and 42 replies, contains a number of similar stories, though the number of MacBook Air owners affected remains unknown.
Apple's MacBook Air update was meant to fix an issue that causes the laptop to take longer than expected to wake from sleep. Also addressed in the software is a problem that could cause the computer's fans to run at full speed after waking from sleep.
Comments
Waiting to speak with an Apple Engineer to help them troubleshoot.
Possibly the SMC reset did the trick but the Air didn't boot up until five minutes later.
Possibly just give the Air some time to rest and dream of electric sheep for 70 minutes.
Mine came back to life after 5-10 minutes. I was a bit worried when it didn't boot up after the mandated reboot, but apparently it just wanted a nap.
Have you checked System Information to see whether you're actually running the updated firmware now? I've tried that process as well, and all an SMC reset does is kickstart the MBA again but doesn't actually install the firmware, as in most logs I've see it'll restore the previous EFI from backup before restarting...
This was one of those rare times I didn't immediately install a firmware update. My 2011-MBA will remain in its current, useable state until this issue is resolved.
This is one area that I think Apple is really dropping the ball on. It seems (to me at least) that the quality of their updates have been lacking lately. This does not help.
On the flip side, the last update for the iMac did clear up an irritating sleep/awake issue finally.
NEVER do an EFI update! NEVER!
I've had two macs get screwed over by EFI updates... never again.
EFI updates, in many cases, deliver tangible enhancement and bug fixes that make your Mac run much better! They've fixed battery, sleep, cooling, and performance issues over the years via EFI updates.
In my years of working with Mac's, this is the first time I've run across this sort of issue.
Same kind of paranoia surrounds PC users not updating their BIOS'.
Don't listen to this guy folks, he hasn't a clue what he's talking about, just spreading FUD. One bad EFI update doesn't mean you should never update again.
Now there's a piece of advice you should NEVER follow!
EFI updates, in many cases, deliver tangible enhancement and bug fixes that make your Mac run much better! They've fixed battery, sleep, cooling, and performance issues over the years via EFI updates.
In my years of working with Mac's, this is the first time I've run across this sort of issue.
Same kind of paranoia surrounds PC users not updating their BIOS'.
Don't listen to this guy folks, he hasn't a clue what he's talking about, just spreading FUD. One bad EFI update doesn't mean you should never update again.
Well, in my (more) years of working with Macs, EFI updates are a crap shoot at best.
I do know what I am talking about as this story attests. Again. Just Google EFI update problem and you can see that this is not the first time this has happened.
So take my word or his word.
Again, EFI updates are released for a reason, they're called bug and stability fixes. They're primary purpose is not to kill Mac's, despite what the tin foil hat community might want you to believe.
Yikes! Now I'm really glad my mid'11 is still running 10.8.5.
10.9 hasn't been a joy on my mid'11 iMac, so I've never installed it on the Air,
despite Apple's fairly persistent 'reminders'.
I would take my word on this, as I've seen hundreds of Mac's receive EFI updates, and this is probably only the second time I've seen an EFI cause any significant issues.
Again, EFI updates are released for a reason, they're called bug and stability fixes. They're primary purpose is not to kill Mac's, despite what the tin foil hat community might want you to believe.
I agree. I've installed hundreds of EFI updates over the years and, luckily, I've never had a bad one. Sometimes they can take awhile... and you must be attentive, do the required steps if listed, and... I don't install EFI updates along with software updates.
They contain a VAST amount of data, so if you're not familiar with it, I wouldn't suggest trying to decipher them on your own
I'm waiting for Apple engineering to call me now to investigate the issue on a few MBA's I have sitting here as Guinea pigs...
I have faced the same issue & I want help to fix it.
I have the same issue.