It has a defect and should not be trusted. Get your files out and contact the maker to get it repaired under warranty.
The Cnet Storage Forum is full of this sort of problems and the answer is simple. Don't trust a flaky unit.
Bob
I have a "Simple Drive" external hard drive with USB port. I have used it on Windows XP Home, Windows XP Pro, and now Windows 7. From time to time, it just vanishes. I have tried unplugging and replugging the USB cord with no luck. In the past, I have restarted each computer and it has come back. Last night, I unplugged the electric cord and plugged it back in, and it came back. This problem has existed on all four computers I've used it on. Is there a way to get my current computer to just be friends with it and remember it?

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