Q My iMac has a small internal hard disk, so I use external drives, one over FireWire, and two connected by USB. Now the USB drives have started to eject themselves, suddenly and without any warning. They usually re-connect immediately, but one has even stopped that, although it mounts normally on my Windows laptop and my MacBook Pro. Why do they do this, and how can I stop it?
A There are plenty of possible reasons, and the best way to distinguish between them is to study the logs in Console immediately after one of these unmounting events.
Otherwise start tracing the fault using a logical sequence. Check their cables thoroughly, preferably swapping out their current cables for known high-quality USB cables. When doing that, assess whether the USB ports at either end of the connection might be loose. Sometimes internal connections have a dry solder joint, which breaks once it warms up but is restored when it cools a tad: an Apple engineer would need to check that, but it should also affect other USB devices. Although normally reliable, USB protocols can get messed up, and it is worth connecting the drives via a modern powered USB hub, and direct to your Mac.
A wide variety of software problems can also cause this. The drives may have developed errors, but before trying to correct them you need to check what their format is, using Disk Utility. FAT32 and other MS-DOS/Windows formats are still better repaired within Windows.
Check for rogue background services and conflicting third-party extensions. If you have other USB devices apart from mouse and keyboard, disconnect them and uninstall any drivers, or start up in Safe mode with the Shift key held down.
Updated from the original, which was first published in MacUser volume 27 issue 9, 2011.