OK I think it's fixed now:
Before, my SSD was fine but my HDD was bricked, and I couldn't boot with both of them connected. I booted with only the SSD connected and made sure that the "Hot Plug" function was enabled in my UEFI. Then connected my bricked HDD which was first recognised as a drive with no storage space (0 GB) and tried a bunch of programs to rebuild MBR, reinitialise, format, DISKPART->clean, clone OS, etc. During that time I had a few blue screens (as I had a bricked HDD and the system complained) but then my HDD was in the state mentioned above.
Since then I rebooted and reseated the SATA and power cables for the HDD and then I could initialise the disk and create a "new simple volume" in my Windows Disk Management.
In my PC the drive is now available and completely empty and it looks like I can move/download stuff into it.
BTW I don't know how to mark this as solved, this is my first time posting.