Palo Alto Products PA-600 ATX Mid-Tower

The two internal 3.5" bays obviously don't make use of the removable drive rails, so what do they use?  The PA-600 makes use of two very low profile internal 3.5" bays, one is located directly beneath the last external 3.5" bay (where you would expect it to be) and the other bay is located on the motherboard tray itself. 

Taking a look at the first bay you immediately notice its low profile nature, however Palo Alto did not sacrifice compatibility in favor of attaining a lower profile as this bay can accept almost any 3.5" hard drive.  The only reason it is dubbed a "low profile" bay is because it isn't a full enclosure, rather it is half of an enclosure with an open bottom (as seen on the right). 

The second "low profile" bay is a little more interesting.  This bay isn't as flexible as the other one as some physically larger drives may have some problems squeezing into the tight area it is forced to occupy.  Depending on how large of a drive you plan on installing you may have to remove the plastic fan holster, however for 99% of today's drives you won't have to even think about that. 

The "bay" isn't really a "bay" rather it is a piece of metal that attaches itself to the drive via two metal protrusions (they fit into two screw holes on one side) and a single screw (the "bay" is screwed onto exposed side of the drive).  After attaching the plate to the hard drive, the plate is then flipped over so it is facing the motherboard tray, and then it slides and locks into place.  As you can probably guess, this isn't the most secure installation, so to make things a little more secure you'll need to remove the front bezel and attach the assembly to the chassis via two screw holes underneath the front bezel. 

The removal of this assembly is quite a bit of a pain since you can't see where the plate makes contact with the motherboard, and it will cause a fair share of frustration whenever you're removing a drive.

Overall the expansion features of the PA-600 are very useful in spite of the actual expansion capabilities of the case being quite crippled.  Unlike the ATCX, the PA-600's 5.25" drive bays aren't enclosed in a removable cage an absence which is made less of a fault due to the presence of the fairly easy to install drive rails.   It all depends on personal preference whether this is truly a disadvantage or not.