That is how DOS always sets drive letters. From Wikipedia:

Assign the drive letter A: to the first floppy disk drive (drive 0), and B: to the second floppy disk drive (drive 1). If only one physical floppy is present, drive B: will be assigned to a phantom floppy drive mapped to the same physical drive and dynamically assigned to either A: or B: for easier floppy file operations. If no physical floppy drive is present, DOS 4.0 will assign both A: and B: to the non-existent drive, whereas DOS 5.0 and higher will invalidate these drive letters. If more than two physical floppy drives are present, DOS versions prior to 5.0 will assign subsequent drive letters, whereas DOS 5.0 and higher will remap these drives to higher drive letters at a later stage; see below.
Assign a drive letter to the first active primary partition recognized upon the first physical hard disk. DOS 5.0 and higher will ensure that it will become drive C:, so that the boot drive will either have drive A: or C:.
Assign subsequent drive letters to the first primary partition upon each successive physical hard disk drive (DOS versions prior to 5.0 will probe for only two physical harddisks, whereas DOS 5.0 and higher support eight physical harddisks).
Assign subsequent drive letters to every recognized logical partition present in the first extended partition, beginning with the first hard drive and proceeding through successive physical hard disk drives.
DOS 5.0 and higher: Assign drive letters to all remaining primary partitions, beginning with the first hard drive and proceeding through successive physical hard disk drives.
DOS 5.0 and higher: Assign drive letters to all physical floppy drives beyond the second physical floppy drive.
Assign subsequent drive letters to any block device drivers loaded in CONFIG.SYS via DEVICE statements, e.g. RAM disks.
Assign subsequent drive letters to any dynamically loaded drives via CONFIG.SYS INSTALL statements, in AUTOEXEC.BAT or later, i.e. additional optical disc drives (MSCDEX etc.), PCMCIA / PC Card drives, USB or Firewire drives, or network drives.

Hmm, didn't realize that was defined behavior. Guess I don't need to mess with it...

It's annoying because it changes the name of the extended partition, depending on whether I boot with the SD card installed, but not that critical in practice.

can pc dos 2000 work with 512 cluster fat16 like NT 4.0 can? If so than you can just make a 4gb fat 16 Drive for C:

There were days when everybody carried a spare hdd in a mobile rack drawer in his backpack for moving data. When I put my drive into the bay of my friend's computer, my drive (one primary partition) would be D: and his logical partitions got shifted (the same problem as yours). One solution/trick was to create a logical partition (or partitions) on my drive instead of primary, so my drive got the next letter after the first drive's ALL partitions (assuming the main drive was primary master).
Other idea is to try to create two primary partitions on the main drive so the SD always comes last, but I am not sure which DOS supports it, you have to try it. (the no-primary-only-logical-in-extended was also illegal in former DOS versions I think).

Other hacks are the ASSIGN and SUBST (maybe JOIN) commands under DOS (Google helps with usage). They do not help with automatic drive letters, but you can create redirected drive letters and directories by software for your needs (or the special needs of some game).

Drive Letter Manipulator

aleksej wrote on 2020-08-22, 13:34:

Drive Letter Manipulator

Cool, thanks a lot. Zip file is mentioned here.