ProRes HQ & USB 2.0

USB 2 has peak transfer rates that are sufficient, but it's not good at maintaining steady high transfer rates - so it'll work for general playback some times, and not others. It also uses more CPU, so it can be affected by other things that are taxing the CPU - like video playback, effects, etc. Lastly, even for just straight editing if you do a transition between two clips you need enough bandwidth to read two streams simultaneously and at 440Mbps for two streams you'll definitely be over the limit of what USB2 can handle. I'd stick with fw800, it has no issues with ProRes HQ for general editing purposes.

Most USB 2.0 harddisks read/write at around 70 - 100 Mb/s.
The USB connection often is not the bottleneck: it's the disk itself.
Nonetheless FW800 or eSata is recommended combined with at least 2 disks in RAID-array.

Always check the write and read speeds of any disk you want to buy.

I disagree, most modern desktop drives can now comfortably exceed the interface speed of even fw800, but the same drive in a fw800 enclosure will perform noticeably better for video work than in a USB2 enclosure - it's the interface that's the problem. ProRes HQ is about 28 megabytes a second - even most laptop/portable drives are fast enough to sustain 2-3x that without an issue. And if you're not doing a lot of work with multiple streams of it at once there's little need for an array.

Hmmm, looks like I've been mixing up bits and bytes.

But:
HQ ProRes = 28 MB/s = 224 mb/s
3 times that = 672 mb/s < 480 mb/s of USB 2.0
Most disks never reach the maximum connectionspeed

A speedtest I just did with an USB 2 WD-disk (85% full) from one of my clients gives read and write speeds of approx. 27 - 30 MB/s. (That 1 stream of HQ ProRes)

My internal Harddrive (connected with eSATA) : 102 - 110 MB/s
(This one is indeed proof that single disks can handle FW800 busspeed)

A quick look at the specs of a LaCie RAID disk (one of few examples where average speed is mentioned next to maximum bustransfer speed)
http://www.lacie.com/products/product.htm?id=10601
tells us there is always a gap between busspeed and average read/write speed.
That gap is somewhere between 20 and 60%.

So, the fact that the disk itself is fast enough for 800 Mb/s is useless when is sends 280 Mb/s over USB 2.0 or 656 Mb/s over a FW800 connection. (The latter is quite good with 80% performance, but RAID0 should be fast anyway.)