External DVD/CD

[Arwen]

In reality, my "important data" is less than 4.7GBytes so I probably could have gotten away with single layer DVD M-Discs.

I haven't looked, do those come in rewriteable?  A multi-session burn will let you keep using parts of it each time, so you could have versioning even without being rewriteable.  You sacrifice a little capacity for that.

The M-Disc BURN data in to non-organic materiel, thus are not re-writable. That is the whole intent, make it very hard to flip bits after a burn.

I don't know if they can be appended to, using multi-session.

My usage of them is archival storage. Burn once, file away.