Win 98 won’t find a USB external HD

I have a Toshiba 6000 laptop (1.2 GHz/256 Meg) which won’t reliably ‘find’ an external harddrive connected via the USB port.
I have been through all the ‘solutions’that I have been able to find – including the usual ‘take away all the drivers’ and reinstall. Plus all the ‘hot fixes’ I can find on the Microsoft KnowledgeBase.
Sometimes the drive is found and works for a short time – then, the next time I restart, it’s not there anymore. The drive light is lit but ‘explorer’ won’t find it.
My older Tosh 4000 (266 Mhz/ 160 Meg)has no problem with the same drive(s) – and it remembers the ISDN cable as well! :o)