my case (A-top X-Blade) comes with 4 open 5.25" drive bays, 2 open 3.5" drive bays, and 5 hidden 3.5" drive bays. The open bays are the ones that you would normally put in the optical drives. in my case thats a DVD/CD/CDRW combo, a floppy, and a media card reader. the hidden bays are what are usually used for things such as hard drives. no point blocking an open bay with a hard drive.