I had an old Samsung 1 TB hard drive lying around, so I decided I’d try to install it in my Librem 13 v2. Turns out the drive was too thick - the SATA connector was too high relative to the motherboard for me to insert it.
FWIW, I was able to successfully install a different 7mm drive (but that drive was defective and clicked a ton so I returned it).
Yes, Librems require a “slim” type hard drive, which these days is much more common than thick drives you described. Could you just note here the thickness of your disk, is it 9 or 15 mm?