X1 Carbon Gen 3 20BS here: It could be noted, that the M.2-connector has very strong spring, so at least my SSD pops up so hard it bounces the screw if not kept hold of. Also the insertion of SSD should be made in same angle as it comes out, not vertically sliding. The type of M.2 SSD is single-sided SATA (double-sided bends the connector too much, not enough space for proper fit), not NVMe.

I could be noted, that the M.2-connector has very strong spring, so at least my SSD pops up so hard it bounces the screw if not kept hold of. Also the insertion of SSD should be made in same angle as it comes out, not vertically sliding. The type of M.2 SSD is single-sided SATA, not NVMe.

The screws are M2,5x6, in case they are ruined as mine were, and they have spacers that prevent them falling out when opening bottom cover.