See also Working with Disk Images
This information is pretty much the same for any type of flash storage.
Save the entire flash (disk) image as a .img file for easy restoration in case of an emergency.
Make sure SSH (server) is enabled on the machine containing the flash memory card. You may also need to open a port on the firewall (port 2222 in this example) if you need access from the Red (Internet) interface.
To back up a firewall/router/appliance from a Unix or Linux machine or Cygwin on a Windows machine issue:
ssh -p 2222 root@ip.of.ser.ver "dd if=/dev/harddisk" | gzip > backup.img.gz
For SSH running on the standard port 22, you can omit the -p 2222.
Change the IP address to your own IP or DNS name
You must adjust the /dev/harddisk to meet your needs. CF cards are often /dev/hda or /dev/hdb. /dev/sda (first SCSI disk) is common for USB flash drives.
If the system you are backing up does not have the gzip utility, use this form:
ssh -p 2222 root@ip.of.ser.ver "dd if=/dev/harddisk" > backup.img
Here is another example backing up a Sonora Advanced Router where the CF card is /dev/hdc:
ssh root@ip.of.rou.ter "dd if=/dev/hdc" | gzip > backup.img.gz
This command utilizes ssh's ability to run commands submitted at the end of the command line. In this case the command sequence uses the dd command to copy the entire physical compact flash device and then put the output into a file on your local computer.