Little Backup Box is a rather unsophisticated script, so it only works when a backup storage device and a card reader are correctly recognized and mounted in the specified locations. The script expects to find the storage device at /dev/sda1 and mounts it in /media/storage. Similarly, the card reader is expected to be found at /dev/sdb1 and mounted in /media/storage. If these conditions are not met, the script won't work properly.
So if you have problems running Little Backup Box, the first thing you need to do is to check whether the card reader and the storage device are detected and mounted correctly. To do this, connect both devices to the Raspberry Pi and run the sudo lsblk command. The output should look something like this:
pi@raspberrypi:~ $ sudo lsblk
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
sdb 8:16 1 29.7G 0 disk
└─sdb1 8:17 1 29.7G 0 part /media/card
sda 8:0 1 115.7G 0 disk
└─sda1 8:1 1 115.7G 0 part /media/storage
mmcblk0 179:0 0 14.5G 0 disk
├─mmcblk0p2 179:2 0 14.4G 0 part /
└─mmcblk0p1 179:1 0 63M 0 part /boot
The important part here is the names (sdb1 and sda2) and mount points (/media/card and /media/storage). If the names are different, you need to adjust the values of the STORAGE_DEV and CARD_DEV variables in the backup.sh script.