Start Lakka in text mode:
Lakka documentation - Accessing Lakka command line interface (under “direct access”)
This will give you a command line. There, you can type “dmesg” to see the system log and look for errors, and when you type “retroarch --verbose”, you should also get an error message that informs you why retroarch could not launch. It’s probably a graphics driver issue. You can also save the output to these commands to a pastebin for others to look at.
Also start Lakka in normal mode and in the debugging shell of mount_storage (screenshot above), please post the output of
mount
mount /dev/sdb2 /storage