My HP dv6000 series does that. Here's how I fixed it.
Before: Computer USB -> USB drive.
After: Computer USB -> USB HUB -> USB drive.
Sorry but you didn't share a make and model so I can't see if there is a known issue with your model.
Bob
Have any of you successfully dealt with this problem? I've seen it a couple of times. I have a Windows XP Home OS computer with SP3 that normally boots fine. But when I plug in my Maxtor One Plus external hard drive the computer will not boot. I've checked bios and the boot sequence is 1st the hard drive and 2nd the DVD drive. There is no option for USB booting so that should be out of the picture. Why would this USB device interfere with the boot? As soon as the computer gets past the system info screen I can plug the external drive in and it boots just fine.

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