Do not try to backup /dev/ram* partition tables.
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This PR fixes a warning on recent Ubuntus:
Warning: The partition table for /dev/ram0 could not be saved.
Warning: The partition table for /dev/ram1 could not be saved.
…You can read more about this https://askubuntu.com/questions/703576/fdisk-l-shows-16-ram-disks-dev-ram0-ram15
Thanks.