Really? Try booting Windows XP from a drive that is not marked as C: somewhere in registry and in config files. Even if you do manage to change the root from C to something else, it simply refuses to boot, end of story. People have tried it, it just doesn’t work. With Win8 and above, yes, it does work, but some programs will out right refuse to work (cuz they’re gonna look for C:\%WINDIR%\system32 for the libs it needs to run, and they won’t be able to find them).
The only exception to this rule is WinPE (for WinXP), and that is a hacky setup, not officially supported by MS. It can be done, but it takes a looong time to actually make it bootable under any drive letter (anything that’s not C).
But yes, you are correct, drive letters were in use before DOS.
Ummm… as I said, everything up until Windows 7 won’t boot, not unless is WinPE based. If it’s a regular install, no, it won’t. Windows 8 and above, yes, it will boot… and it won’t throw errors. Will everything regarding 3rd party software work? No. Can it be fixed? No, no source available. Then what’s the point of booting from anything other than C? IDK, you tell me 😂.