I have RB493G with RoS 5.26. After an upgrade to 6.10 lost card MicroSD. in System-store - no discs. I rolled back to 5.26 card appeared.
Probably a bug
I have updated and rolled back several times - the same result
Remove the card from the router board.
Start the router board back on.
Stop the router board.
Plug the card back in.
Start the router board back on.
see if it works now.
I did all strictly on the recommendation, but it did not help.
maybe a problem in the firmware (v6)
I tried v6.4,v6.6, v6.10,v6.11rc, but did not help
maby a bug report this to Mikrotik
make supout.rif and sent this to support@mikrotik.com