Several external drive options exist at different levels of performance, capacity, and redundancy. One of the oldest types of externally attached subsystems used SCSI drives. Outboard SCSI RAID arrays are still used for certain mission-critical applications mostly in businesses. The SAN, or storage area network, offers another enterprise-class external storage solution. While personal SANs may someday make it into our homes, for now SANs are still considered high-end and beyond the scope of this piece, which focuses on personal external storage.
A few external IDE storage products have also been on the market, but havent been completely satisfactory. They have been, for the most part, not hot swappable, so you typically had to power down to swap out a drive. And you were still limited by the number of free IDE ports on a system. The most common variant has been drive bays that slide into the PC case and plug into an internal connector.
IDEs short cable length (24 inches, tops) has been the major limiting factor for IDE-based external drives. However, as Serial ATA begins to take off, well see more external drives, because the maximum cable length has increased to 1 meter.
After FireWire arrived on the scene, FireWire external hard drives used as backup or secondary storage became a familiar item, especially in Macintosh shops. On the PC side, IEEE1394 was less prevalent, as the standard has only recently become popular in most PCs and motherboards. Now that PCI firewire cards cost less than $15, these offer another viable option. But USB 2.0 is a standard feature on all current generation motherboards, so it obviates the need to install a PCI card.
Finally, a few external drives existed for USB 1.1 connections, but the maximum transfer rate of 12 megabits per second was too limiting for serious data transfer.
Once USB 2.0 hit the streets, USB became an interesting option. Weve seen a number of external CD and DVD recordable drives support USB 2.0. We reviewed several examples of these back in August. External hard drives have been somewhat slower in coming, but now theyre arriving in force, and most of the major hard drive suppliers now sell external drives supporting either FireWire or USB 2.0 -- or both.
While buying a drive thats already integrated in an enclosure has its appeal, you can also build one yourself. That way, you can choose the capacity and performance you need.
Unlike SCSI, however, external USB and FireWire drives wont match the performance of an internal drive. A SCSI drive is a SCSI drive, whether its external or internal. However ATA drives today all support high speed DMA transfers, while USB and FireWire do not.
Because of this, you might conceivably see performance differences that depend on the speed of the host CPU. Since you typically wont be using an external drive as primary storage, the differences shouldnt matter. What it does mean, however, is that you can get away without using the fastest possible ATA drive.
We deliberately chose a newer drive, the 7200rpm Seagate Barracuda ATA V, which we reviewed in our big drive feature. It wasnt the fastest in our roundup, but if we hit any performance limitations in USB 2.0, it should show up on this drive -- and we also get to toss in a comparison with our recent Serial ATA test. We chose to use a USB 2.0 external drive enclosure kit from Belkin.