Last night Apple pushed a small OSX update, purportedly related to Firmware. It required a reboot and completed successfully. Unfortunately, it also stopped one of my external USB disks from mounting. Others were mounting fine, but this disk (which was mounted the night before, during the upgrade, and removed after shutdown) couldn't be seen.
To say I was scared to death is an understatement: there's always a chance that problems like these are due to hardware issues. Unfortunately, Solid State Device (SSD) disks have a limited life and will eventually fail. This was a 500 GB disk with a lot of very useful stuff on, programs I need every week or so, now-untrovable applications, etc. Before I'd let go of this baby, I wanted to go to the bottom of it.
Almighty Google pointed me to an old post mentioning USBProber but it didn't mention what USBProber is (a debug tool by Apple) nor where I could find it (it's not installed on OSX by default, at least not on 10.8 Mountain Lion). Luckily, another blogger mentioned it, and I could finally get it from the Apple site: you will find USBProber at the Apple Developer Site under the name "IOUSBFamily Log". I got the 10.8 version and installed the package ending in "-Log", as instructed.
USBProber.app can be found under /DevTools/Hardware. Once launched, it gives you a number of options to track USB activity. I'm not a hardware hacker, I only have a cursory knowledge of Plug and Play mechanics from my old Linux days, so I poked and prodded here and there without much luck. Eventually, what seems to have done the trick was to select "Bus Probe", ticking "Probe suspended devices", and then hitting "Refresh" several times, until the drive appeared in the list. Switching to Finder, I could see that the drive was in fact mounted, and all contents were displayed. Phew.
I hope this is not something I'll have to do over and over. Hopefully I'm not the only one to see this issue, and Apple will eventually push a correction. Meanwhile, USBProbe looks like a nice find to troubleshoot problems like this, and hopefully this post will be useful to people in the same situation.