I've been working with Nimble snapshots and I restored a previous snapshot of a volume that is a datastore in my ESXi 5.1 environment. When you restore a snaphot Nimble has you take the volume offline. It's a good idea so you don't shock the filesystem you're working with, in this case VMFS.
So no problem, I'm expecting my datastore to go offline as well as my virtual machines that are built from it. I bring the volume back online after the snapshot restore and my datastore just continues to show Inactive... No amount of refreshing, rescanning and pleading will bring it back! The weird thing is it's showing active in the detailed storage adapter view. I have it mounted on another ESXi server and it's showing showing up just fine! What the heck?!
So I check the /var/log/vmkernel.log and I find, "Device is permanently unavailable". Oh no what the heck did I do?! I'm thinking I've got some weird iSCSI duplicate ID's, or something weird.
Then I remember my friend Occam's Razor. What's that you ask? Basically all things considered, the simplest explanation is usually the answer. I remember I was doing some CLI work on this ESXi machine and I check to see if I'm sitting in a directory that is no longer mounted on that machine. Sure enough that's where I was! I change directory out of there and guess what? My datastore came back. :-)
So what does this teach us? NEVER use the CLI! NO, no, just kidding. If a problem seems totally bizarre and won't resolve itself, usually it's the simplest explanation.