Using Polyserve clustering technology from HP, you need to ensure to have Automount off.

This is done using diskpart from a command prompt.

DiskPart

Automount disable

If this is not disabled you end up with "phantom" drives, and it can cause problems with everything from virus scanning, back up software and windows itself !  Basically what happens is the SAN Administrator zones new storage, somewhere the server reboots (got to love that!), than you have a new drive letter, but basically it is a "phantom" and netbackup, virus scanning or some other program attempts to access the new volume through it's drive letter, and the software abends....nice !

Removing the drive letter is usually a straight forward process of using Disk Management or Veritas Enterprise Disk Manager and removing the drive letter, but I've had issues with this and had to drop do a command prompt and use DiskMount to remove it.