Commodore wanted new C64 drives to be backward compatible with VIC-20 and vice versa. They failed the second goal, and C64 sold ~10x the number of units VIC-20

Commodore wanted new C64 drives to be backward compatible with VIC-20 and vice versa. They failed the second goal, and C64 sold ~10x the number of units VIC-20 did making whole exercise pointless. All to sell more outdated garbage chips made by MOS instead of using proper FDC controller on CPU bus with cheap standard floppy.