Thanks for the update, this indead sound really strange. I take it you did not remove the sdcard between Android and the flashing of SFOS (just to really rule out any connectivity issues)?

Regarding the output of dmesg (and other logfiles): Unfortunately, SFOS does produce quite a lot of output, especially when developer mode is enabled and you are connected via SSH. I suspect the interesting parts of the output, what happened during the mounting attempt (if any), are not shown via the tai commandl (which by default only outputs the last 10 lines).
Would it be possible that you post the whole output of dmesg during the mounting attempt? issue the command immediately before inserting the sdcard. From the output, note the number at the beginning of the last line in square brackets (that it the current uptime in seconds as time stamp). Then, attempt to mount the sdcard and afterwards issue (without tail). Copy all the output starting from the time stamp of the previous command. This way, we can see what happened in between the two points in time.
Since this is probably a lot of output, may I ask that you copy them into a text file and use it as attachment here? The output should not contain any sensitive data (like phone numbers, names etc.) but please feel free to remove anything that looks like that before posting here. Thanks.