ALL drives will be monitored by System restore. Even storage drives for some reason. Is it not possible that somehow the drive simply "lost it's letter" from disk management?
1. Start->Run "Diskmgmt.msc"
2. Look at the bottom pane. What items are present?
You can add/assign a drive letter on your drive:
type the command above, press Enter....
Computer Management window appears..
1.Right click on the "partition" of your disk (drive w/ no letter assigned)...
2.Click "Change drive letter and paths"...
A new window appears...
3.Click "Add" button..
A new window appears...
4.Click mouse on "Assign the following Drive letter" option...
Select letter you want to use on the drop down option....
5.Click OK twice...
Check if new Drive letter (the one you specified) appears on Disk Management window...
You can try open My Computer and access your drive if you can....
Hope this helps......
P.S. Pls. post if you Guys, have corrections on my reply... I'm a newbie, ya know...