External USB drive won’t mount on OS X? Try safe boot.

I like to reuse old computer parts where possible. I’ve got an old 2.5″ hard drive rescued from a laptop headed for the junkyard Recyclinghof in a USB enclosure that is serving Sarah’s much more modern laptop quite well as a back up driving, using Time Machine. We’re using the same drive, but it’s partitioned into two different Time Machine volumes.

"Timemachine gallery windowsquicklook20070611". Via Wikipedia.
Timemachine gallery windowsquicklook20070611“. Via Wikipedia.

But try as I might, I could not that make that drive mount reliably on my machine. Until now.

Symptoms

I could plug in the drive into one of my USB ports and the green drive light would go on and you could hear it whir to life, but it never mounted. The System Information and Disk Utility never showed the drive. And yet it mounted quickly and painlessly on Sarah’s Macbook Air (a couple years older than mine, but running the same OS version).

Sometimes I would leave the drive connected (but not mounted!) overnight and come back to find that it had eventually mounted and the Time Machine backup had run. But usually not. I thought it might be a question of the cable, the connector pins, even something mechanical about the drive itself. One time it came to life while standing vertically, so for weeks I thought that must have something to do with it — kinks in the cable or drive inertia or something.

Solution

It’s working now, but I still don’t know what the problem was or why the fix worked. All I did was start up the machine, a late-model Macbook Air, with a safe boot, while the drive was connected via USB, after googling for similar problems. I am not aware of any other problems on this Mac, so I figured a safe boot couldn’t hurt. It’s “safe,” right?

  1. Turn your Mac off.
  2. Turn it back on, and press and hold the shift key (either one? I used the left shift key) as soon as you hear the chime.
  3. It’ll check the startup disk, and take longer to boot than normal.
  4. When you log in, not all the usual stuff that happens upon login will happen. But that’s when my drive started happily blinking away, and I saw it was mounted and ready for business.

Safe Boot

Here’s what Apple says about starting your Mac in “safe mode.” Nothing on that page really leaps out at me with a solution that indicate external drives connected via USB are handled differently, unless maybe the failure to recognize and mount the drive was caused by an unnecessary kernel extension, and disabling it via the safe boot made the drive usable again. But if that’s the case, then a subsequent normal, non-safe (um…unsafe?) boot should have caused the drive to fail to mount again. But it doesn’t. So safe boot must have fixed something else.

I kinda want to know what it did, but I’m just glad I didn’t junk this drive without giving it one more try.