RESOLVIDO!
Refira Como congelar o drive no Linux? para resposta.
Success!! I managed to unfreeze my drive by making the SATA port it is connected to hot-swappable. This involved me going into my BIOS. How you can achieve this will vary depending on your BIOS manufacturer.
Once the SATA port was hot-swappable, I did the following: 1) turned pc off 2) disconnected the SATA data cable from my motherboard. Left power cable connected. 3) turned PC on and booted into Linux 4) Connected HDD and Viola! It was unfrozen and "sudo hdparm --dco-identify /dev/sdb" was working! 5) I checked to confirm it was unfrozen by running "sudo hdparm -I /dev/sdb"
Note: device is at sdb now and not sda (as you will find in my other thread - linked above) because the HDD was connected after Linux was booted into!
If you cannot make your drive hot-swappable, you will have to buy a SATA-USB cable and connect your drive via USB.