I've already written one post that went into detail about reformatting an external drive that acted as my Time Machine backup for my Mac. External USB drives can be bumped and the cables loosened, raising the chances that data corruption will occur; encrypted drives are really cranky when that happens.
In my case, reformatting and starting over is acceptable for recovering and getting the system backed up again as long as there isn't any indication that the hardware itself is failing.
This time around any time I tried accessing the data was met with failure; from the diskutil command line utility, it seemed that there was an encrypted volume being found and mounted by the system (even though it wouldn't appear in Finder nor in Disk Utility...Disk Utility kept hanging with a spinning beach ball and wouldn't show any drives mounted at all when it launched...). Diskutil seemed to show the existence of an unlocked encrypted volume on my Time Machine drive, but it had no data, and attempts to reformat the drive using the command line returned an error to the effect of "resource busy."
After several attempts to read the volume I decided to try just obliterating the data on the drive by wiping as many sectors as I could at the beginning of the drive. Unfortunately, attempting to do that returned a resource busy response. A daemon on OS X was trying to access the drive, and that prevents direct access to the device.
But there is a way around it.
Use this command to identify your external drive (the Time Machine drive, in my case)
diskutil list
Su to root.
sudo su
Disconnect the drive from the USB port. Then use the up arrow and enter key to repeat this command when plugging in the drive again. The goal is to execute this command just after the system sees it, but before the auto-mount daemon tries to be helpful and prevent your access...replace "disk1" with with the correct disk number found with the diskutil command above. Triple check that you have the correct drive. If you overwrite the wrong drive you will be very unhappy and it's not my fault.
cat /dev/random > /dev/disk1
This will overwrite data on the drive with random gibberish. Once enough sectors...like the partition table...the drive will be seen as ready to be formatted by OS X. This process is not going to give much feedback...and since the drive is large, it could take forever to complete. I advise letting it run for several minutes then use control-C to abort the command.
At this point I used Disk Utility to format the drive, then opened Time Machine to remove the previous backup drive and re-add the "new" one.
And yes, I re-encrypted it. I'd rather not let the backups be readable by others, despite the risk of corruption wiping the backup, and creating a new set of backups took only about a day to complete.
