I had a 1571 disk drive to go with my Commodore 128. It cleverly emulated the 1541 when in C64 mode, including the absolutely atrocious speed, and the save-with-replace bug. :( Not that being able to overwrite a file is a basic tenet of disk-drive operation, or anything. Hmm... maybe CBM stol the BIOS off a tape-drive, which didn't generall have to worry about such hoity-toity shenanigans as overwriting a file...

On seconds thoughts the monitor may have been a 1901. Since I also had (no, have!) a 1081 and a 1084S, no wonder I get confused.