Using External Harddisk as an internal desktop SATA Harddisk

It seems like the SATA-USB card in the external case encrypts data. So, the hard-disk is hardware encrypted, and can not be decrypted using any software. I am saying this on the basis of the discussion here:

http://www.tomshardware.com/answers/id-1828742/book-3tb-external-internal.html

So, the hard-disk can only be decrypted by that SATA-USB "card". If I do want to use it as an internal hard-drive, I will have to re-format it while it is connected to my internal SATA port.

I have no experience with exactly your MyBook, but I used a couple of those in the past and I clearly remember them having the disk divided into partitions mounted in a RAID-1 array. This will most likely befuddle your system, if you do not have the package mdadm imstalled. I would try that,

   sudo apt-get install mdadm

and then restart gparted.