MM - thanks, but I'm 'OK' with DLA.
Jim - thanks also. It is always 'screechingly obvious' to those who fully understand DOS/Windows, but as one who teeters around the edge I am unsure. Yes, I'm 'OK' with the principle of the BIOS boot assignment but:
1) Do I need to re-letter the existing C Drive partition (to 'U:', say) before unplugging it and consigning it to the great dustbin of hardware?
2) If I do not, will Windows change all the drive letters itself to re-letter the new boot drive as 'C:'? I am never sure of the significance of the letter 'C:' in the system but I'm sure I recall it has some system significance.
(I have a Hiren boot USB if I do need to 'fiddle around' outside the installed Windows)