Interesting. Plenty to keep me going tonight when me brain won't shut down.
Saab I think I had tried that, but tried it again just to make sure. Still writing pages of data in Disk Monitor, sometimes 100 lines in a second. Several hundred in a burst. The it goes quiet for a few moments.
One will remove the original HD tonight, to purify the logic a bit.
The other thing is Check Disk being called every start up...and one time hundreds of pages of something were erased then seemingly replaced. I thought that would be the end of it, but wishful thinking I'm afraid.
I think there is no point in too much further work until I've taken Occam's razor to the drive set up.