Sometimes we get fixated on specific settings and possibilities when troubleshooting problems with our Macs, and in doing so might overlook other seemingly unrelated settings that might end up being the root of the problem at hand.
Recently, MacIssues reader Graham wrote in, asking about a problem with his Thunderbolt drives unmounting at random intervals:“I bought a LaCie 2TB (2 disk) drive last week and several times it reported that it was not ejected properly…I moved up to a WD 4 TB My Book, again Thunderbolt connected, but when it also reported a failed eject, I repartitioned into 2 disks and just left it connected. I came back a couple of hours later and once more – Failed to Eject properly. I tried different ports. I wiggled the cables while connected but this did not indicate any weakness. I am guessing power saving may be a problem, but USB (2 and 3), Firewire 800 (with Thunderbolt adapter) do not show the same problems.”
Spontaneous ejects of external media happen when there is a severing of the connection between the drive and the computer. This can happen from the use of faulty cables, incompatibilities between daisy-chained devices, and improper power management for attached devices.
To tackle this problem, Graham noted that using a Thunderbolt-to-FireWire adapter does not cause drives to eject, and since this uses the same Thunderbolt port on the computer, it was unlikely that the cause was from power mismanagement. Instead it pointed to either the Thunderbolt cable or a problem with how the Mac is handling Thunderbolt connectivity.
Upon trying a new Thunderbolt cable, the problems persisted for Graham, and it looked like the obvious solutions were not applicable in this case. While further troubleshooting might have gone in the direction of reapplying updates or reinstalling OS X, our discussions regarding power to the devices had Graham try toggling some of the system’s Energy Saver settings.
In the Energy Saver system preferences, you can check the option to put hard disks to sleep when possible, or enable power nap on systems that support this feature. Graham found that by checking the option to prevent the computer from going to sleep when the display is off, the problem no longer occurred.
While this is not a fix for the problem, it does at least keep it from happening, and allows the system to work without ejecting attached drives. Another possibility is that this error is related to a faulty System Management Controller or PRAM settings, so resetting the SMC and resetting the PRAM may help for others who experience similar issues. Otherwise, hopefully this type of error can be fixed in a future update to OS X.
Graham has written an extensive report of his drive troubleshooting on his blog.