This happened to me recently, and a helpful guy at os2world solved this intricate issue.
I was experiencing growing problems with my USB thumbdrives. I couldn't understand what was happening, since everything was working perfectly before.
In the meantime however I had been regularly upgrading the ACPI drivers, and it turns out that if you have an nVidia-based (nForce4 or 5) motherboard and you have installed ACPI drivers in the 3.x version numbers, you're very likely to have this problem.
Pete solved this problem by adding the "/CD" switch to the ACPI.PSD line in config.sys, and so did I.
Don't let the docs fool you: even though they say the "/CD" switch is to be used on SMP systems only (APIC mode), it worked perfectly on my single-core box in PIC mode.
I am happily using my thumbdrives again! ;-)
