I have it all in a spreadsheet with serial number, original cost, current market value replacement cost, and a photo of the instrument as well as a photo of the serial number. Also each guitar has a unique identifier somewhere on it, not all the same identifier and not all in the same place, that only I know about. That info is on a CD locked in a fireproof safe as well as stored in a few other places.

I remember a guy telling me about his carefully prepared inventory stored on a CD but his CD was in his home office. I had to ask if your house burns down, do you think it will burn everything but that one CD? He now has other copies elsewhere.