I had the same problem. Don't know why!?!?!
I recently bought 2 x 250gb WD hard drives in external USB enclosures. Let's call them drive A and drive B. I have a Dell 8300 running XP SP1 with all updates, 6 USB ports, 4 back, 2 front. Latest BIOS flashed.
I plugged drive A into the front port, new hardware was detected, new drive detected. Went into Disk Management, partitioned and formatted the drive, assigned drive letter. Great, everything works, copied some files to and from the drive to double check. Sounds good, right? Turned off, unplugged drive from USB port.
I did the same with drive B. Worked.
A few days later, I come back, plug in drive A. No hardware detected. Not listed in Device Manager. No sign of it. I reboot a couple of times with drive attached. Nothing. Unplug and replug. Nothing. Check connections and change cable. Nothing. And then I try unplugging and replugging again, and wouldn't you know it, hardware detected, etc ... listed in My Computer, works perfectly now. Then I turn it off, unplug it.
I then try drive B. No hardware detected. I try everything I tried with drive A, can't get XP to detect it. I give up.
I try drive A again. Works first time. Good, but weird.
Anyone have any idea as to what is going on?

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