Lessons learned

Build notes

The gotchas, so they don't bite twice.

Drives & booting

Drive-letter trap: booted from USB, USB = C: and hard disk = D:. Don't format C: blindly — confirm by size (40 GB disk shows ~38000 MB) and prefer the guided installer.
Install each OS with the others unplugged so each bootloader stays on its own disk. Reconnect after; pick with F8.
MBR, not GPT: DOS and XP need MBR. The old openSUSE left a GPT "protective" partition that had to be deleted before FDISK could make a DOS partition.
SATA IDE mode works only on SATA1–SATA3 on this board — keep drives on those ports.
USB not in boot priority? Insert it before entering BIOS; it shows under Boot → Hard Disk Drives — promote it there first, then in boot priority. Or just use F8.
FreeDOS partition size: answer "use maximum size = Yes" or it can default to ~1 GB.

FPC "invalid zip archive": corrupt package + unset TEMP. Fix — set TEMP to a writable folder, remove the half-registered package, reinstall from a good zip. Verify with dir C:\ppc386.exe /s.
Package target: the package manager installs to wherever DOSDIR points, not the drive you type. Set DOSDIR to the target disk, or install while booted from it.
XP ISO hash: verify before burning. Genuine XP Home SP3 SHA1 = 5A6B959AD24D15DC7EBD85E501B83D105D1B37C6. The first download failed (a repack) and was discarded.

"Chassis Intruded!" — harmless; the case-open switch tripped. Clear it in BIOS (Power → Hardware Monitor) and disable while building.
One PS/2 combo port: keyboard or mouse, not both. Keyboard takes PS/2; the serial mouse goes on COM (needs a COM bracket — the board's serial port is a header).
USB Legacy Support = Enabled so USB keyboard/mouse work in DOS and BIOS.
Monitor: use VGA — it shows BIOS/DOS legacy modes cleanly where DVI can be fussy.
CPU support: Athlon II X2 245 needs BIOS 1303 or newer.