Is there any way encrypt my NAS with an external program?
You can set up an encrypted container in the NAS, with the decryption being done on the client PC. Only the PC has the encryption key. Generally this only works in cases where there is only one user at a time.
An example of this is Microsoft's virtual disk (vhd and vhdx file extensions). These containers can be created on a PC, and then moved to the NAS (and accessed over the network). Alternatively, they can be used exclusively on the PC, but then dismounted and copied to the NAS for backup.
Encrypting on only one pc is not an option. Is there any other way? What is usually the best practice?
Just to clarify - the encryption would be done on every client PC. The restriction is that only one client can access the secured data at a time (though multiple clients can access it if you are careful). You can of course have multiple containers (one for each client, or possibly for different categories of data).
What i need to accomplish, is to secure that if someone steals the NAS he won't have access to my files...
This is easier said than done.
Your NAS can't do volume encryption, though a new OS-6 NAS can.
With volume encryption in the newer NAS, the key is stored on a USB flash drive. That needs to be inserted when the NAS is booted, but it can be removed after startup. You'd need to remove it, and secure it separately since if someone steals both the NAS and the key, then he still has access to your files. But if you do that when there's a power failure, etc when you aren't home, then you'll lose access until you can put the USB key back into the NAS.
The limitations here definitely get in the way, and security isn't guaranteed - since no matter what you do the key can be stolen too. Plus there are other threats - if the NAS is running, then network security is also needed, since the volume encryption isn't end-to-end. And volume encryption won't protect your backups, since the NAS doesn't support encryption on USB drives. Of course, I have paper records with private information too, so even if my digital data is fully locked down, the threat of identity theft after a robbery remains.
So I've chosen to live with some risk there. My OS-6 NAS can do volume encryption, but I don't use it. There are some files that I keep on a Microsoft virtual disk (as outlined earlier). Those are secured on my laptop (since I think laptop loss/theft is more likely than home theft), and the vhd containers are backed up to the NAS.
