Use ECC DDR3 with that Supermicro board.

Yes. The X10SLM-F supports ECC, and for a backup server you want reliability over saving a few bucks. Non-ECC might work, but why risk random bit flips corrupting your backups when ECC is cheap and easy to find used?

That board uses DDR3 RDIMMs or UDIMMs with ECC. Used ECC DDR3 is plentiful and cheap — often cheaper than non-ECC desktop sticks because it’s less popular for gaming. Your backup server will sit there silently 99% of the time, but when you need it, you need it to be correct.

Non-ECC would probably boot fine, but it’s the wrong tool for the job. Don’t overthink it.