|This happened to me when I tried to add a secondary IDE drive to a computer that came out of the box with a primary SATA drive. Turns out that Vista somehow thought my IDE drive was an "EISA Configured" drive, which is normally used for OEM System Restore info. The Disk Management snap-in prevents you from doing anything to any partition that it believes to be a System Restore partition.|
The only way I found to fix it is to use the DISKPART command-line utility to reset the partition type back to a normal NTFS partition, not an EISA (restore) partition. As an added bonus, this will leave any data that was on the partition intact. Here's how:
Use at your own risk - Microsoft's help file for the SETID command suggests that only OEM's and IT Pro's should be meddling with this. You can mess up your partitions if you use the wrong ID! This walkthrough assumes your partition is NTFS (code 07) - if not, go look up the right code and use it instead!
Start > All Programs > Accessories > Right-Click Command Prompt > Run as Administrator
SELECT DISK [id of the disk with the errant partition]
SELECT PARTITION [id of the errant partition]
(Confirm that the ID is 27. 27 is the hex code for OEM reserved partitions.)
(07 is the hex code for regular NTFS partitions)
You can now use either DISKPART or the Disk Management snap-in to assign a drive letter to the newly unlocked volume.