@x56h34
Yes, I seem to have got it sorted, thanks to your excellent advice. It seems the selection of 'Use' for the amended FastATAPrefs is more applicable here than the 'Save' option.
I couldn't get HDToolbox to run after booting from my Emergency Disk, so I tried just clicking on the 'Use' option under standard Workbench. This popped up a warning that I was about to modify the drives' existing settings and did I wish to proceed? (Never seen that before). Then, after OK'ing the warning, I loaded HDToolbox. This then showed the new drive as a single entry, instead of the nine or ten logical units I was seeing before (as created by the FastATA 'Split' function and which seemed almost impossible to avoid).
I then divided the drive into the larger units I wanted and formatted these with SFS filesystem. On rebooting, there were still icons for the original unwanted 'Splitted' logical drives, so I cleared their RDBs using the old HDInstToolbox. On rebooting again, all the dross disappeared and I had the big logical drives I wanted. Great!
The FastATAPrefs system seems not to be very friendly to say the least! I'm blowed if I can understand the manual on this issue and I'm awaiting Elbox's suggestions on the problem.
Anyway, I seem to have got it fixed and learned a little bit more on the way. Many thanks.
JaX