i purchased an WD external hard drive about a month ago, I also own a teac 40Gb external hard drive. They were all working fine until a couple of days ago.
My computer has stopped recognising the correct hard drive size. I'm not talking about a small percentage of the drive but in the case of the WD 160gb of size. When I right click 'properties' it says that I have 160gb of free space (it also says this in disk management). Yet when i try to copy something it says that the disk is full. The same thing has happened with my 40gb drive. I connected both drives to my girlfriends computer and they worked fine.

As far as I know I have windows service pack 2 so I'm completely baffled as to why this has happened. Please help

I have two suggestions for you; 1. If you have system restore activated on your computer, use it to before the problem occured. 2. Reinstall drivers (by cd or through the manufacturors website). Good Luck!

As unlikely as it may seem, a corrupted MBR or MFT could cause this issue... At least on a single drive.

I would try the drives on another computer to see if they work properly. This would be a good way to determine whether or not your computer has somehow damaged the partition tables of these drives. If the drives work, then that's good news and we know the problem is not what your computer has done to the drives, but how your computer is reading the drives.

I don't really know of a good answer off the top of my head for this one though. System Restore is a good shot. You can also try the drives in safe mode to determine if it is a software problem or your partitions are muddled.

update usb drivers
make sure the drives are not attached
check the cables
check for write protect on the drives
if nothing else since your GF's machine see's them and they function correctly save as much data as you can and reformat
It may be a write protect thing
can you read any of the data could be read only ticked by mistake

I've checked the hard drive on my girlfriend's computer and it recognises the space, I've also run a scan disk to fix errors on it to see if that works (on her computer).

System has been turned off for a month because I had a previous virus problem so that cannot restore the drive to its original settings.

I will try your suggestions and get back to you. Thanks for the help

If you had problems caused by a virus then your best option is to reformat and reinstall windows. seond best option would be to attempt a repair install of windows.