If you are trying to mount an SD card that already has files on it, be sure to back up the SD card before proceeding.
If the London isn't recognizing the SD card, it is possible that the memory card might not be correctly formatted. Go to: Settings > Storage > SD Card Section. There you will find options to mount or erase the SD card. First try mounting the SD card. If still not accessible and you have backed up all the data on your card (if any) try to Erase the SD card. This will format the card correctly and should be mounted automatically.
Other steps to try:
1. Insert the SD card into your computer
2. Format the SD card:
* For PC computers, make sure the format setting is set to NTFS.
* For Mac computers, make sure the format setting is set to ExFAT.
4. Eject the device from your computer.
5. Insert the SD card into the Marshall London.
6. Go to settings
> erase SD card
> mount SD card
It may help to label the SD card: please label the SD card while it is loaded into the computer. If necessary, there are 3rd party Android apps that will allow you to label and SD card while it is mounted in your phone.