That’s what I do. I upgraded the SSD in my Steam Deck to a 1tb (before the prices went nuts, thankfully) and then ended up with a spare 256gb drive. I put it in an enclosure and installed NixOS on it as a sort of second system. It doesn’t really do anything super important, it’s more a learning project to get comfortable with NixOS without interfering with my daily driver. I could have just as easily done it in a VM but what can I say, I’m a tinkerer lol