When I connect a USB pen drive in my computer it randomly disconnects. I already unchecked the power manager option in the device manager. I have the same problem in XP & Windows 7. I tried with Ubuntu on the same machine without any problems.
Had a similar problem with a combination of usb card reader and old CF cards.
Basically issue was the card didn't allow simultaneous read writes. But somehow the system did try to do it - which caused it to immediatly eject.
After turning off thumbnails, indexing and also taking care I did not do any browsing on the card during read/writes I could use it "normally".