It's not quite that simple. You have to do something like
dd if=/dev/zero of=/media/Big\ Bertha/extimagefile bs=1M count=1 seek=50000
mkfs.ext4 -F /media/Big\ Bertha/extimagefile
That's what I mean by 'make an ext4 filesystem image', before
mount -o loop /media/Big\ Bertha/extimagefile /media/backup
Run the above commands before you run osmc-backup and make sure you can
ls /media/backup It's better not to cd /media in fact. And yes, -p is a good suggestion. The above will make a 50GB 'sparse' image ie one which actually takes up only as much space as its contents need. This may have been the reason for the 'painfully slow' remark. You may do better with count=50000 and no seek parameter.