My iMac DV 400 boots into OS 10.3.9 and OS 9.2.2, and I have no problems with USB flash drive recognition. For power reasons, it's better to only plug the drive into an unused USB port on the computer itself, and not to the keyboard's USB (mouse) port. If you've connected an unpowered USB hub to the iMac's USB port for multiple devices, you should use a powered hub instead.
Have you tried more than one flash drive? Have you tried the drive on another computer?
Occasionally I hear of one type not working. I have two retail PNYs and one generic thing with ads on it I was given at a trade show. All three work on all our Macs running OSX, although I've not tried it on with our iMac 400 DV booted intoOS9. Will try later and report back..
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Imac G3 and flash drives
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