Hi All. This is my first post, so if I screw something up, have mercy.
So anyway, I had the error "HostStorageSystem.UnmountForceMountedVmfsVolume failed" when I tried to unmount a snapshot volume after I had finished with it. I double checked that there weren't any VMs still pointing to it so I was a bit perplexed why it wouldn't unmount. When I mounted it in the first place, I had followed the KB article entitled: Force mounting a VMFS datastore residing on a snapshot LUN results in the error: Cannot change the host configuration (KB Article: 1015986). But, I discovered that there wasn't any information about unmounting a force mounted LUN when finished with it. No searches of VMware or the 'net via Google yielded anything but API references for the method that failed.
Here's how I fixed it, in case you have the same issue: I had enabled I/O Control on the volume while I was using it. Once I disabled I/O Control, I was able to use the GUI to unmount it. Anti-climactic, I know. Hopefully someone benefits from this info.
Cheers,
-dp-