So, I'm running a xcopy with about a dozen parameters to copy the entire directory tree over to some space elsewhere. In theory I could just fdisk the disk as it's basically a two-year-old snapshot of my data, but I've been bitten before by abusing fdisk, which resulted in a partition being used for scratch space going bye-bye, and me a few days later realising that actually, some of the stuff there was rather useful.
And now it appears to be copying over various source trees (currently mozilla 1.4a, by the looks of things), which being full of little C files are going at about 1 megabyte per second. Compared to earlier transfers with larger files, where it peaked at over 12 MB/s (running disk-to-disk on the same ATA100 bus). Next will be to zap that partition, then re-run PartitionMagic and start juggling drives.
On the plus side, I've been fairly hammering the I/O for a good while now, and the music from XMPlay has been almost rock-steady (only a couple of slight glitches).