Since you’re already on btrfs it’s probably better to actually use the features it has to expand your existing filesystem to cover both drives.
But if you just want to mount it somewhere and not worry about figuring out anything complicated, what I did is mount my big new cheap disk at /home/bigdisk and then have symlinks pointing to it such as ~/Videos -> /home/bigdisk/Videos
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Huh. So definitely not worth it then. Just buy a space heater instead. When I did it a few years ago I got $500 out of it, but it cost me a $350 video card which died soon after.