I have an older 2TB Seagate drive that was constantly on the fritz when running under Windows, but since getting my MBP it's been well-behaved. Since shooting weddings is such high-volume work I didn't want to get my primary MBP HDD bogged down with tens of thousands of RAW files, so I've recently moved them all to the USB 2.0 Seagate external via the Lightroom Library module. Since Lightroom needs only to access the files to render previews or to export, this setup works perfectly and I'm in the process of migrating all of my RAWs to external drives now to free up space on my onboard HDD.
FW800/Thunderbolt/eSATA would be wonderful for file transfers, but really once the data is on the external drives, the throughput of those interfaces doesn't really matter (at least in my experience thus far) when you're doing your editing.
That said, transfering ~32GB worth of photos every Saturday via USB 2.0 is getting old, so I'm in the market for some high-capacity Thunderbolt drives...
Can't wait for the iMac refresh with USB 3.0 AND T-bolt

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Canon EOS 5D MkII | Canon EOS 7D | Canon Digital Rebel XSi | EF 35mm f/1.4L | EF 50mm f/1.8 II | EF 85mm f/1.8 | EF 100mm f/2.8 Macro USM | EF 135mm f/2L | EF-S 18-55mm f/3.5-5.6 IS | 580EX II | LumoPro LP-120
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