Simple way to backup your device data is to create image (.img or .iso) file. You can even create image file for your folder.

To view device file of SD card use below command

fdisk -l
diskutil list

Lets say if your SD card is /dev/sdb then you can create image using

dd if=/dev/sdb of=./SDC_Backup.img bs=4096

If you want to restore data then you can use same command but with input and output swapped

dd if=./SDC_Backup.img of=/dev/sdb

This is very simple way to backup your data. You can even create image for HDD data.