Any Mac veteran who fondly—or perhaps not-so-fondly—remembers the first iMac and the Power Mac Cube—can’t resist a transparent case, since it lets you see a gadget’s innards. The OWC Mercury-On-The-Go Pro’s clear plastic case provides a full view of the drive within, as well as the electronics surrounding it.

The Mercury-On-The-Go Pro drives were among the only 320GB drives we tested, because they allowed us to compare the performance difference between a 7,200rpm and a 5,400rpm drive in the same enclosure. As the performance results show, 7,200rpm definitely helps, but primarily if you’re using FireWire 800 as your data connection. When testing performance through the Mercury On-The-Go Pro’s FireWire 400 and USB interfaces, the difference did not matter much, and other lower-rpm drives proved to be superior performers. In addition to its cool see-through case, the 5,400rpm 320GB Mercury-On-The-Go Pro is the least expensive in its class; the 7,200rpm drive costs quite a bit more, and is only really relevant to FireWire 800 users—and even then the performance difference is relatively small.