Family photos sound like they should be in scope.
And here’s the beauty: it doesn’t matter where they are, or how infrequently they’re accessed. It’s exactly the same problem as your question about using both a work computer and a home computer – the answer is the index.
Put these photos on whatever external/cloud/offline storage device you like, and in the index entry, just tell yourself where they are in whatever words make sense to you.
Also check out this recent post about items that span servers and drives which covers many of the same themes.