I don't think it was spinning at the time but I can't be sure. However, plugging it back in I found that it wouldn't mount. I tried a few times, tried using USB but it refused to mount. All the spin up, head alignment noises were there but it didn't mount.
Running Disk Utility showed an "Invalid content in journal" error. Now this wasn't a FCP drive, just a storage drive of mine. It's good practice to not journal your render drives (if you use an external one) for reasons such as this, speed, etc…
But this storage drive was journaled and apparently something went poof when its firewire was unceremoniously unplugged.
Running repair in Disk Utility showed:
The volume BLAHBLAH was repaired successfully.
Volume repair complete.
Updating boot support partitions for the volume as required.
Repair tool completed
But Disk Utility lied. Running Verify or Repair showed the same "Invalid Content in Journal" error every time. And it still refused to mount.
I decided to give Disk Warrior a go even though my version is out of date and after it's 10+ step process of rebuilding and replacing the directory the drive mounted.
Out of curiosity I rechecked it with Disk Utility and found no errors but I did notice that apparently Disk Warrior had turned off Journaling on the external drive.
Just to see what would happen I re-enabled Journaling through Disk Utility and re-ran Verify and again, no errors.
So if a drive won't mount, and you see the "Invalid Content in Journal" error in Disk Utility give Disk Warrior a try. Looking back I wonder if I could have just disabled Journaling on the un-mounted drive and then mounted it.