I tried to secure the data of the hard disk connected to my Airport Extreme, for example in case of theft. So I started looking for solutions, but on the disk shared with the Airport Extreme, there is not much solutions. One solution I used over the course of a year was to create encrypted DMG files (Mac OS Disk Image), which is not an excellent solution at all. Finally, I found a simple solution that fully met my expectations. It was a 2.5-inch hard drive enclosure that performs hardware-level encryption

Finally I found something, you can have a look for a solution among others : Here

Or smaller : Here