@Ego what do you mean with “the encryption can only work on a completely empty drive” ?
If I make two or more partitions, are there problems ? With my FDE I did this : /boot on USB pendrive and three partitions on the HDD that contain OS and other data .
@tomorrow LUKS is the encryption method, you can erase the HDD with shred -6 -uvz or other methods, but each HDD could have its recovery method/tecnology .
Of course you can’t use the SSD or USB for security reason, the trick is to use these storages only with FDE, so when you delete the key from the USB the data are lost forever, at least I hope 
. In this case you should destroy the pendrive when you stored the key and then, for extreme security, also erase the SSD/USB storage .
In HDD case is almost the same but the problems are less .