This drive was set up with a Dynamic Drive Overlay.
I originally posted the problem here:
garycase suggested nuking the drive with dban (Darik's Boot & Nuke - use with care)
This worked, I had 40GB back, points allotted to garycase with grateful thanks - problem solved.
Some time later I needed to reformat and the drive suddenly reverted to 23GB. This time dban won't do the trick. I'm tempted to try the old marine engineer's "float test" - throw it overboard.
Seagate Barracuda 7200.7 IDE drive.
Is dynamic overlay controlled by firmware on the controller?