How to rescue Unsupported Format Variant files?

[stangame]

Hi support, I have read some other queries on this topic in the support archive. My message relates to UFV's in both jpg and nikon raw NEF's. The files (as far as I can guess) corrupted in the last few weeks long after they were ingested but probably while I was re-keywording them in PM. They will not open in any viewer programme including File Browser in Photoshop and Windows Explorer. Some are completely invisible to the viewer, others show the UFV graphic. The corrupted files are stored on an external usb hard drive.
System is as follows:
MSI Pentium 4 2.8ghz, 1.5gb ram.
Windows XP home SP2 with all hard drives in NTFS format, OS not refreshed for 2 yrs approx, but auto-updated.
Photo Mechanic 4.5.3.2
Windows running AVG 8, Zone Alarm, Windows Defender, Quicktime 7.2.
Ext. hard drive: Seagate USB 250gb device, approx 30gb free.

Regardless of how I improve my system to avoid this in future (check USB drivers, only one high speed device per root hub etc) how do I rescue these files now?
Regards, Stangame.

[stangame]

DISREGARD PREVIOUS MESSAGE. I restarted my system and connected the external hard drive to a different port and the problem has gone. Handy, since my files in other folders were vanishing before my eyes and panic attacks were imminent! Now to carry on keywording and backing up to dvd....(yawn)
Regards, stangame.