I recommend this method for making a backup of your flashdrive. The method is reliable but the result is not an ISO file.
Open a terminal and type the command:
You have to identify the device assigned to your flash drive. I will suppose that this is /dev/sdc (change 'c' to 'b', 'd' or other letter if necessary).
You have to umount the two partitions of your flashdrive now. Type this other command:
You will see the two partitions if they are mounted. These two command will umount them:
Finally this command will make the backup of your flashdrive:
The file containing the backup is named flashdrive.dd.gz. Use another name if you want.
sudo dd if=/dev/sdc conv=noerror,sync | gzip > flashdrive.dd.gz
This is the restore procedure.
Insert a flashdrive of the same size or bigger than the original flashdrive. I will suppose that its associated device is /dev/sdc (use command sudo fdisk -l).
Unmount the partition(s) of the flashdrive if they are mounted (use the command umount if necessary).
Finally type this command:
gzip -d < flashdrive.dd.gz | sudo dd of=/dev/sdc conv=noerror,sync