Why does every attempt to fix Windows/GRUB boot issues inevitably end up with me running `dd if=/dev/zero of=/dev/sda bs=446 count=1`...

This time Windows was convinced I'd installed it using EFI so wouldn't let me access the MBR repair tools (probably because GRUB2 was using EFI and chain-booting to Windows?). So to fix it I needed to reconnect the DVD drive that's been disconnected for 12 months, nuke the MBR using SystemRescueCd, boot into Windows recovery and bootrec.exe /alltheflags.

I'm glad I didn't go with my gut feeling of bs=512 (despite making a backup) which also would have wiped the whole MBR, including partition table.

Wow, all these years I thought the bootstrap code was 448 bytes, not 446. Considering I never broke anything, I clearly got lucky all those times.

TIL.

I did a double-take when I saw the 446 too. There's a few places online that use 448, which I can only guess is due to people thinking 64 bytes for partitions but neglecting the 2-byte signature.