Am having to work on this very thing this AM and just discovered a little thing to help see what's going on in the build area of the backup.
In moodledata/temp/backup/
while watching Moodle do it's thing issue the following watch command:
watch "ls -lR -I '*.log';df"
After the ls -lR that's a dash capital "I" ... not a lower case L. I stands for "ignore'.
That will ignore the .log files and show only the contenthashed named directory Moodle is using to build the backup. At the very top of that contenthashed directory, the .mbz file being built will show 'progress' ... ie, you'll see it increase in size.
You could drop the ;df as that would show disk free output and it's assumes you are working with a LARGE disk. Screen clip follows:
'spirit of sharing', Ken