Even though the host has already been unregistered, the local datastore for that host still appears in the vSphere Client.
The host experienced a hardware failure, causing virtual machines to fail over. The problematic host was then unregistered from vCenter Server.
vCenter Server (All Versions)
A virtual machine that was failed over to another host had file information on the local datastore.
ex: virtual floppy file
Edit the virtual machine settings and remove the floppy information.