Cant access both drives

I partitioned the drive in half.  I used partion A to back up computer A.  I used partion B to back up computer B.  Now when I plug the WD Smartware back into computer A it automatically recognizes partion A, but drive B is no where to be found.  I can understand Smartware trying to guess which drive to connect to (although it guessed wrong), but why cant I see drive B under my computer (using Windows 7 64bit)?

I’m a bit confused on this one, do you have the option on the HOME tab to change the destination and source drives? o.o

No, I dont have that option in the HOME, on computer A. But when using computer B, I saw both drives under “my computer” so I could choose which drive to use.  It seems now that I have chosen a drive to backup computer A with, it no longer recognizes any other drives.  I thought I should be able to share files between all drives, like as if I just had a computer with 3 hard drives?