One of my pooled drives is flagged with a "Pending drive xxxx" message, how do I fix this?

If one or more drives is fagged with a "Pending drive xxxx" (or similar state), you need to edit the drive's state to fix this issue.

Add a drive letter to the drive in question... do this by showing advanced options, then Map drive letter.
 
 
 Then go to the root of the drive (i.e. using Windows Explorer), we need to edit the "PI" file. Note the actual file name will be different on your system.

Right-click the file and open with notepad (or a text editor of your choice).
 
Change the "State" value to "0" (zero), save the file then reboot.
Once the system restart, the drive should be flagged as healthy.