XPsp3 FORGETS I’ve got a flash drive plugged in.

I have 2 Sandisk Cruzers (8GB). When I plug them in they are recognized immediately but a few hours later I get a “device not present” message when I try to use them (I’m only using one at a time for back-up but have the same problem with each).
I go through the “safely remove hardware” routine and see the drive light turn off, unplug and replug and they work fine. I am able to store data on them and have reformatted several times.
wb8mwr