harddrive not mounted?

A friend of mine is having an issue with an external HDD. When he connects
it to his computer, it doesn't show up under My Computer or anywhere in
Windows Explorer. His message to me said:

"I am wondering if it happen when I was restoring my computer to a previous
date. I unplugged my [external] harddrive during this process and got the
message that said:

Changes made to drive H after this point cannot be reversed because the
drive was either excluded from System Restore monitoring or was turned off or
removed."

The drive does show up in the Disk Management utility and does have a drive
letter assigned to it and says it is healthy and active. He says that when
he connects the drive, the computer slows down dramatically and he has tried
it on various computers, all with the same result.

When we used the "mountvol" command from command prompt, the drive's GUID is
reported but instead of listing the drive letter, we get "The system cannot
find the file specified." I really don't know what that means. Here is a
screenshot of the results of MOUNTVOL:
http://www.techimo.com/forum/attachment.php?attachmentid=13603

Would running CHKDSK help this at all or is there something else we could
do? This one really has me stumped. Thanks in advance.