While covering for a colleague at one of our clients, I noticed that one of the datastores was showing as inactive on one of the hosts (ESXi 5.5 Express Patch 10, 3568722)
After rescanning the hosts, and then trying to unmount the datastore it had become clear that it was an issue with a single host as all the other hosts had successfully unmounted the LUN.
When I tried to remove it from the troublesome host, I got the error message:
After trying to unmount it via by PowerCLI
Remove-Datastore -Datastore Datastore -VMHost 10.23.112.234 -Confirm:$false
I also tried via ESXCLI to stop start storage IO
# services.sh restart
# /etc/init.d/storageRM stop | start
..all was unsuccessful.
In the end, a host reboot resolved the issue. Sometimes, the simplest answer is the quickest answer.