DFS Read Only
The *DRIVE command in the 1770 Acorn DFS has an option parameter after the drive number for the drive size, 40 or 80 track. Drive 2 here is an 80-track drive, as we can see from the free space map, so the drive is read-write when we first select it. Selecting again, but setting it as a 40-track drive, which has the effect of double-stepping the head, to allow 40-track discs to be read in 80-track drives, makes it read-only, so the second save operation fails.