Back in 2008 I took a NSLU2 with SlugOS/BE and made sortof a backup appliance with an encrypted disk, using
The NSLU2 has two USB ports and the idea was to attach a disk with an encrypted filesystem to one port and (temporarily!) an USB-stick with the key to the other port.
The USB-stick with the key needed to be plugged in only when the encrypted disk needed to be mounted. Once it was mounted, you could unplug the USB-stick-with-the-key to keep it at a safe place.
The appliance could stay at a (physically) unsafe place and you could store your backups on it. If the disk was stolen, it couldn't be mounted without the USB-stick that held the key.
By the way: did it actually work?
Nope, not really. The NSLU2 was to slow. Its CPU went to 100% whenever I only threw a glimpse at the disk.
I was able to scp files to it, but the rate was much too low to get reasonable amounts of data onto the encrypted disk.
But almost 10 years passed since then. So, I bet it will work with nowaydays single board computers.
I just dug up the (brief) documentation I made..it's in german but if anyone is interested, i can translate it.
But then, after a very brief googling, I guess there are much better documentations, like this one.