I tried a USB2.0 hard disk that I have previously been using under winxp. I noticed the disk was exceptionally slow so I looked in /proc/bus/usb/devices and it was listed as a USB1 device. Then I got some explaination: (from syslog)

Mar 14 04:34:08 localhost kernel: hub 2-0:1.0: over-current change on port 1
Mar 14 04:34:11 localhost kernel: usb 2-1: new full speed USB device using uhci_hcd and address 4
Mar 14 04:34:11 localhost kernel: usb 2-1: not running at top speed; connect to a high speed hub

The disk works on the same computer under windows xp (but sometimes xp too complains about too much power usage so I guess my computer is really picky (it's a laptop) but xp just warns, the little balloon disappears after a while and everything is ok). Anyway, in xp it works, in linux it does not. Also, I don't really see the correlation between the first message about current and the last one about speed (other than the obvious empirical one, that is).
Just to clarify, I have been using a lot of other USB storage devices, and they work without problem (that is, usb2 devices work at full speed). And yes, I have ehci_hcd loaded.

My guess about this whole thing is that the hard drive requires some amount of power to build up angular momentum on the disks. When the disks are spinning, power settles at a lower level. This power fluctuation maybe somehow confuses the driver into thinking we're dealing with a USB1 device? Which doesn't help me much right now. In a desperate attempt, I tried unloading uhci_hcd, but that was a bad idea, as I lost bluetooth and the mouse (and the hard disk too for that matter).

Help! Copying ISO images at ~600 kb/sec is just unbearable.

You can increase your hard drive performance with hdparm although I don't know how this article would integrate with a USB drive. Good luck with it -- J.W.

Thank you, that article was really interresting. Although I get about 40 MB/sec on my laptop HD already. However, it has nothing to do with my original question I think.

I just noticed something: If I boot into windows xp when starting the computer and then reboot to linux, everything works:
/proc/bus/usb/devices
....
T: Bus=01 Lev=01 Prnt=01 Port=00 Cnt=01 Dev#= 2 Spd=480 MxCh= 0
D: Ver= 2.00 Cls=00(>ifc ) Sub=00 Prot=00 MxPS=64 #Cfgs= 1
P: Vendor=04cf ProdID=8818 Rev=a3.01
S: Manufacturer=Myson Century, Inc.
S: Product=USB Mass Storage Device
S: SerialNumber=100
C:* #Ifs= 1 Cfg#= 1 Atr=c0 MxPwr= 10mA
I: If#= 0 Alt= 0 #EPs= 2 Cls=08(stor.) Sub=05 Prot=50 Driver=usb-storage
E: Ad=03(O) Atr=02(Bulk) MxPS= 512 Ivl=0ms

It now reads Spd=480 (before it was Spd=12). Also, it is listed right below the ehci_hcd driver (before it was below the uhci_hcd driver). Is there any way to enforce this set-up so that I don't have to boot windows every time I'm gonna copy files...