If you have a look at file_system you will notice a couple of partition, that contain data, that is not part of OpenWrt and probably unique to your device. To be on the safe side, you should create a copy of this data, on your PC.
Once again, please have a look at the Details and take notice, that the bootloader partition and
NVRAM/etc. and similar partition are NOT part of the OpenWrt firmware. So if something should go wrong and the data on these partition gets corrupted, you will not be able to replace it! You would have to go to the forum and ask somebody to make a backup of his, send it to you, you would have to replace the MAC address and then flash it via port.JTAG since your device would probably not boot any longer.
As already stated, these partition are not part of OpenWrt, and so we do not write to them, and thus it is very unlikely, that they get corrupted. But it is not impossible, and you should make a backup.
If your ART-partition got corrupted, you would still be able to boot OpenWrt and only your wireless would not function correctly any longer. Easy fix with
dd if=/dev/$(grep '"art"' /proc/mtd |cut -c 1-4) of=/tmp/art.backup
If your bootloader-partition got corrupted, you would not even have a booloader console which you could only access through Serial Port any longer, and the only way to recover from this would be though the JTAG Port or by de-soldering the flash-chip, but please see generic.debrick for help.
However, once you've gotten yourself into the position to write to the flash again, you will still need something you can write to it. Something that will work. And here is, where your backup will come in handy:
dd if=/dev/mtd0 of=/tmp/boot.backup
Then copy your backup-file via scp or ssh to your PC and keep them safe for the time when you may need them.
Sometimes it might be necessary to backup settings/partitions from original firmware. Depending on the bootloader different strategies might be possible.
The flash-chip is mapped to a start adress. With uboot it should be in settings
Many bootloader allow you to work with mtd partition, but beware: they do not have to be identical with the Kernel mtd partitions! Also, with some bootloaders, you cannot use mtd-partition, you must work with offsets. In the latter case, it is probably a good idea to write down these correct offsets when you make the backups.
mtd write art.backup artAbove method could work, but most probably will not as art partition is usually not writable, so you will have to compile you own kernel after doing some minor modification like described here: TL-WR1043ND or here: https://forum.openwrt.org/viewtopic.php?pid=135278#p135278. Then you must flash this to you device, boot it, and now the partition should be writable.
You can backup configuration through the web interface, at System:Backup / Flash Firmware:Generate Archive. If the web interface is not available, connect using SSH and enter:
sysupgrade --create-backup /tmp/backup-`cat /proc/sys/kernel/hostname`-`date +%F`.tar.gz; ls /tmp/backup*
Now on your local computer, use scp to retreive the file: (Remember to use the ip of your OpenWRT device and the filename of your backup file.)
scp firstname.lastname@example.org:/tmp/backup-openwrt-2014-08-26.tar.gz .
You might also want to backup other files you have edited such as: