"...somewhere in the dim and distant past I read that Windows would not recognise certain letters. I think the examples given were F and G"
OOoops! Forgot to mention LASTDRIVE! (introduced in DOS 3.0)
DOS 6.x only assumes 5 driveletters (A to E) unless the LASTDRIVE specifier is used in CONFIG.SYS
SYNTAX:
lastdrive=[letter] - (don't use a colon)
OR lastdrivehigh=[letter]
where [letter] A-Z (default is E and A to E/F are ignored here)
EXAMPLE:
lastdrive=K
If the DOS command is used to manage the upper memory blocks (it usually is), the values for LASTDRIVE are automatically loaded into the HMA - (LASTDRIVEHIGH is invoked without needing to be explicitly stated).
If an invalid character (such as a trailing colon) is included, the minimum number of drives (A to E) are included. No error message is displayed.
The reason for LASTDRIVE is of historical interest only. Each additional drive above E: takes up 81 (some say 96) bytes of system RAM and in those far off real mode 640K limit days every byte saved could make a difference.
Windows 95/98 assumes LASTDRIVE=Z, though you CAN change it if you really want to (why bother). XP accepts the specifier for compatibility's sake, but ignores it.