Equipment:
Dell 8300
Disk 0: 30 Gig PATA (Drive Letters L and M)
Disk 1: 250 Gig SATA )Drive Letters C thru K)
Boots from the C: Drive

Problem:
Drive Letters L and M are not available in the DropDown List for the Cache File Location

On a Related Note:
Drive Letter M: on the PATA drive is listed on the left side of the Configuration Panel (as though it were a memory module). See image included.

I discovered this problem when I was unable to designate partition M: on the PATA drive for the Cache File Storage Location. I use the M: Partition on the second hard drive for my windows pagefile and Ramdisk storage file as well.