Same thing here. I just bought a X-Craft enclosure and added an HD into it.
Works well on my PC with XP (transfer a 700Mb movie in less than 1 minute) but the transfer is so so long on my iMac (takes about 15 minutes to copy the same movie).

I've tried to plug both USB cables, but it doesn't work better. On my PC only one USB cable is enough to make it work correctly.

Somebody tried to contact coolerMaster (or Apple ?) for that problem ?

If you're using USB and any device on the same bus is USB 1.1, then all devices on that bus will operate at USB 1.1 speeds. The speed of USB 2.0 of up to 480mbps is approximately 40 times the 12 mbps (approximate because those are peak speeds) so if it takes a little over 15 minutes on the Mac and around 30 seconds on the PC, the Mac might be getting stuck with USB 1.1 speeds.

Yeah, I know. But the problem is that I don't have any other device plugged on this bus.

And the problem only occurs with the X-Craft. When I try with another external HD (MyBook 500GB), the transfer is fast (recognized as a USB2 device).

I'll try with a Windows XP installed on Parallels to see if it's a software problem (with Mac OS X) or an hardware problem(iMac) with that specific external drive.

it's definitely not a problem with the bus, because MyBook drive works well.