Basically put, the PHR-100AC is the Big Daddy of the PHR-250OTG 2.5" enclosure. Much larger in size, more aluminum for extra heat dissipation, and just more of everything come to think of it. Firewire and USB2.0 speeds are available. Just holding the PHR-100AC for a moment I got an idea of how serious macally regards enclosures. The enclosure is rugged and tough, ready for any hard drive you might have in mind.
Since the PHR-100AC is independently powered, you do not have to worry about taking away precious power from your computer system. Hard drives are one of the top computer devices which love to suck power out of your computer system. Externally-powered hard drives, enable you to enjoy the storage freedom you've always wanted, without immediately upgrading your power supply just because you wanted another hard drive.
Macally's PHR-100AC also has dual-firewire ports for connecting additional firewire powered hard drives and enclosures if necessary. This is very convenient in case you don't have room on your PCI USB2.0/Firewire card (and I'm guessing you probably don't).
If smaller storage is not your current goal or a necessary component of your computer system, then having a 3.5" external storage device is certainly going to be a great alternative to having several internal drives. And since the PHR-100AC is a 3.5" enclosure, your external storage capacities can get quite robust and large at a significant cost break versus mobile hard drive external storage. Taking advantage of the latest 3.5" IDE drives, you can have several hundred gigabytes and even half a terabyte of data.
Don't Toss Your Unused 3.5-incher
So, you just bought that 300GB with a wonderful rebate and now you're thinking of selling your outdated 120GB on eBay or giving it to someone. Don't do it just yet. Simply buy an external enclosure unit and you've got not only a portable data storage system, but a great back up system as well.