Shirt Pocket Discussions - Working hard drives not recognized

I have been backing up my iMac to a 500 G Maxtor successfully (I think although I've never needed to find out!). I recently have added 4 separate 1 TB drives chained together. They've been accepting files fine. I have one TB for iPhotos and i for iTunes. Now I want to have SD backup each of these drives. My iTunes is on a Buffalo DriveStation. I copied my iTunes to this drive and set iTunes to use it. Now when I ask SD to back this up to WD MyBook (I know you don't like them but I didn't know that when I bought it!) SD doesn't see my Buffalo drive. Clicking on the drop down selection for what disk to copy from it's not there! Ohhh, Ohh, god. I see. I just right clicked on the icon and it tells me it's formatted for MS-DOS. Damn! I have 490 gigs on there now! Do I have to get them off and reformat? Ohh, my. I'm gonna have to by ANOTHER drive! HELP. But how does this happen that I can use an MSDOS drive on my Mac?