There are 4 internal 3.5" bays total, according to Best Buy's website. HP's website appears to be woefully lacking any useful information. With one taken up by the pre-installed hard drive, I believe you should be able to add 3 additional drives. I have not yet upgraded mine, so I'm not 100% positive. If it's very important that you have more than a couple of extra drives open, it may be worth a call to the manufacturer to verify.