So after much deliberation on an external, I recently purchased an OWC Mercury Elite Pro which tends to get rave reviews from the Mac community. My primary use will be for my Lightroom library and I bought it specifically because it has Firewire 800. However, I decided to first use it to take a Lion "testdrive" only to find out that it will not boot over Firewire (800 or 400)! It will over USB2 only. This is very disappointing and their advertising is extremely misleading in this regards. I went with OWC because the drive can be easily upgraded in the future to "grow" with me. I currently have a different drive serving my Time Machine, but that is an older 500GB Maxtor One Touch that will be reaching capacity in then not too distant future. That drive only has FW 400 however it boots just fine. The OWC drive does work with Firewire 800 as a regular external storage drive, but it will not be able to be used as a future Time Machine enclosure, at least not over Firewire. My iMac is a little older - a late 2007 aluminum C2D iMac, version 7,1. In the little booklet that came with the OWC drive it does show a list of boot options for different versions of current Macs and yes, my 7,1 iMac is listed as USB only. However, that information is not available anywhere on the OWC site and not made available prior to purchase. The online version of the manual does not include this compatibility table. Apple also doesn't list any issues with my iMac with regards to Firewire compatibility, saying all Macs with a built in Firewire port support booting from an external Firewire drive. And yes, I made sure the drive has the proper GUID partition. So, after all the great reviews and recommendations for OWC enclosures, I am very disappointed. I was specifically looking for a Firewire 800 drive.