I was recently fiddling with my external drive and retroactively encrypting the partitions. I have two on there, one from my previous 2010 macbook, and my current 2015 macbook. For the 2015 one, I followed CCC directions and encrypted by using it as my startup disk and turning on FileVault2. For the 2010 version, since I am not using it as a bootable clone (I couldn't even if I wanted to because of hardware incompatibilities... unless I really had a strong urge to), I read that I could just encrypt it by right-clicking it from my desktop and selecting "Encrypt".
The problem is, I was spaced out and ejected the entire disk while encrypting the 2010 partition (2015 was completely finished). I have no way to tell if it was still encrypting but it was a couple minutes after it told me to restart to continue with the initial (2010 partition) encryption. Even though I didn't encrypt the FileVault2-way (again, for the 2010 partition), I presume it still takes a considerable amount of time. But, there's not indication of when it's done! Now, when I eject it, the disk won't stop spinning.
The good news is, I never physically unplugged the disk from it's USB connection. So perhaps it's still doing it's thing behind the scenes? Please help.