I run mirc 6.16. on windows 2k sp4, without problems, including on an external hard drive, no problems at all, this looks more like a problem specifically to your machine. (This is a laptop that I use)
My machine specs are:
Compaq Armada 7800
Windows 2000 sp4
256 megs of memory
the hd is a western digital 200 gig drive in an external case using USB.
I've been running it this way for about 6 months now without one issue.
