Here's how to back-up data to BD M-Disc. This assumes we have a M-Disc compatible rewriter.
Create an ISO image of your data directory that you want to burn.
$ genisoimage -f -J -joliet-long -r -allow-lowercase -allow-multidot -o <image.iso> <dir>
Mount the image with mount -o image.ios /mnt/cdrom to check that all data
and files look good. If not careful with genisoimage flags it's possible to
get truncated 8.3 formatted filenames.
Burn the image with growisofs -speed=1 -Z /dev/sr0=image.iso. Make sure to
set the speed to one so that it does not mess up large BD M-Discs. Some
people were saying that higher speed options may work but it may incorrectly
write data to M-Disc since they like slower writes.
At this point we should have an archived copy of your data.
Burn time estimates of ~20GB iso image: