modeseven

I once had a 27” aluminium body iMac with this exact problem. Random USB disconnect events affecting any USB attached storage drives. It only manifested itself over extended periods, like an initial time machine backup to an external USB disk. Incremental backups would rarely be affected. Hilariously, I spent ages trying to figure it out myself, to no avail, despite being computer literate and working in the Tech industry. I eventually emailed Steve Jobs about it, and then got great attention from a bunch of people at Apple. They never diagnosed the fault. Eventually, years later, I finally figured it out. I would occasionally get mild electric shocks from the aluminium chassis. I assume this was a design flaw, or some errant connection was being made because something had come loose inside and was making unintended contact within the chassis of the machine. It was some kind of electrical/earthing problem with my iMac. If you read around, USB is apparently quite sensitive to issues like earthing problems. My natural assumption was it was a software fault, but in the end I decided I had been barking up the wrong tree. My guess is this was either a hardware/design problem, or a faulty device off the manufacturing line, and an errant electrical contact was being made which was periodically affecting the operation of the USB bus.