Okay, had a brief power outage today that shut down my computer and external drives, so I figured I should run a quick disk check just to be safe.
However, when I run try to verify with Disk Utility, one of the volumes (an encrypted Fusion Drive) is getting stuck on the "Verifying storage system" step. I've tried leaving it, but even after more than eight hours it wasn't showing any progress of any kind.
I've tried using both the Disk Utility app as well as the diskutil verifyVolume command line utility, but nothing happens.
I've also tried doing this with the volume not being used as the system volume, but it makes no difference; not sure if it's relevant, but even though the volume isn't the system volume, Disk Utility isn't unmounting it to verify it. It only just occurred to me now, so I'll try and remember tomorrow to keep the volume unmounted and run a check in that state to see if it makes any difference.
I'm not even sure if I remember seeing the "verifying storage system" step before, or if it does come up it must scroll away quickly normally, certainly never had it stick before.
Best Answer
So I tried running disk verification with the volume unmounted and verification occurred without getting stuck as before.
You can unmount a volume from Disk Utility (don't eject it), but if it's your system volume then you'll need to boot into the recovery partition (hold Command + R during start-up) as the volume won't unmount otherwise.
I still can't determine why verification is getting stuck for the volume while it's mounted, or why Disk Utility isn't unmounting the volume automatically, but all tests are returning success so at least the file-system appears to be okay.