What "dmesg" command shows, when you open & close cover? Should be sth like that:

[97938.596374] mmc0: cover is open, card is now inaccessible
[97938.906646] mmc0: card 1234 removed
[97940.222595] mmc0: cover is closed, card is now accessible
[97940.743347] mmc0: host does not support reading read-only switch. assuming write-enable.
[97940.743377] mmc0: new high speed SDHC card at address 1234
[97940.763427] mmcblk2: mmc0:1234 SA08G 7.41 GiB
[97940.763702] mmcblk2: p1 p2 p3