1. Use the rear motherboard mounted USB ports. USB ports on the front or via those wire cables often have issues.
2. Look for the Intel's latest motherboard chipset driver package (at intel.com)
3. Consider the age of the machine. Power supplies get electrically noisy and we hope for a total failure since explaining this to customers is nothing short of a lesson in electronics when they just want it fixed but don't want to replace power supplies, motherboards, etc.
My desktop (Abit IP35 Pro MB, Core2 Duo, 4 GB RAM, WinXP Pro SP3) has two external USB drives: a) Seagate FreeAgent 500 GB; b) Maxtor oneTouch4 Plus 750 GB. Problem: On occasion after boot up either one or the other of these drives aren't visible in My Computer or in Disk Management. When either or both drives are visible, they function correctly, i.e., the contents are accessible. I'm stomped as to what the root cause of this problem could be. All OS patches and virus/malware software are up to date. Thanks for your support.