Early Mac users often suffered from "Disk Swapper's Elbow" — a painful problem during disk copying.
Why It Happened
- Macs had:
- 400KB floppy disks
- 128KB RAM
- Only one floppy drive
- To copy a disk, users had to:
- Eject the original disk
- Insert a blank disk
- Format it
- Drag files over
- The Finder copied data in small chunks, forcing disk swaps.
A 400K disk needed 5–6 swaps (okay). But sometimes? Over 20 swaps (frustrating!).
Users hoped the 5th swap was the last. If a 6th came, they panicked. By the 7th, they cursed — stuck in a loop.
The Technical Problem
- The Finder used 46K code + 10K overhead.
- Only 30K RAM left — too small for smooth copying.
- Solution: Split Finder code into:
- Tiny copy-only part (loaded first)
- Rest of the code (flushed out)
- This freed ~75K RAM — worked usually.
But sometimes, the system reloaded the big code chunk, fragmenting memory → more swaps.
The Hidden Bug
- The bug was rarely seen in testing.
- Users reported:
"Copy took 20+ swaps! Tried again, worked fine." - Why? Experts didn’t make the mistake causing it.
The Mouse Mistake
- New users struggled to drag icons with a mouse.
- If they accidentally dropped a disk icon mid-copy:
- Finder recorded the new icon position.
- This required code from the big chunk.
- But that chunk was supposed to be gone!
- Finder tried to call it → memory broke → more swaps.
The team never saw this — they were mouse pros, never dropped icons.
The Fix That Broke It
- This bug was added 2AM before launch.
- It fixed a worse bug, so it was the lesser evil... really!