Encryption: What Happens If The Device Is Stolen?

So I followed the instructions posted at My Cloud: Encrypting a RAID Volume and now all my data is encrypted (RAID 0 24TB EX4). But I note that I can download the encryption key when I am logged on. Does this mean that anyone who steals the physical device can discover the key on the HHDs if they invested enough time looking? I know you’d ordinarily need to log on and use the Dashboard, but presumably, someone could even us a brute-force attack to login (given enough time) and download the key at which point they have access to the data - no? Thanks for any advice or pointing me at any materials to read?!!

DOES ANYONE KNOW THE ANSWER PLEASE TO THE ABOVE, i.e. nyone who steals the physical device can discover the key on the HHDs if they invested enough time looking? I know you’d ordinarily need to log on and use the Dashboard, but presumably, someone could even us a brute-force attack to login (given enough time) and download the key at which point they have access to the data - no? Thanks for any advice or pointing me at any materials to read?!! Thank you