I can reproduce the problem and I can confirm that Mike Gabriel's fix fixes it.
Before the fix, I get this (and I can reproduce this over and over again, it's been happening every night for weeks now): david@silverstone:/mnt$ sudo /usr/local/bin/mount_backup_usb.sh && sudo rsnapshot -v daily echo 31032 > /var/run/rsnapshot.pid /bin/rm -rf /mnt/BACKUP/daily.6/ mv /mnt/BACKUP/daily.5/ /mnt/BACKUP/daily.6/ mv /mnt/BACKUP/daily.4/ /mnt/BACKUP/daily.5/ mv /mnt/BACKUP/daily.3/ /mnt/BACKUP/daily.4/ mv /mnt/BACKUP/daily.2/ /mnt/BACKUP/daily.3/ mv /mnt/BACKUP/daily.1/ /mnt/BACKUP/daily.2/ /bin/cp -al /mnt/BACKUP/daily.0 /mnt/BACKUP/daily.1 /usr/bin/rsync -a --delete --numeric-ids --relative --delete-excluded \ --exclude=/home/*/.cache/ --exclude=/home/*/.local/share/Trash/ \ --exclude=/home/*/.local/share/gvfs-metadata --exclude=/home/*/.mozilla/ \ --exclude=/home/*/.cxoffice/**/Recent/ \ --exclude=/home/*/.cxoffice/**/Temp/ --exclude=/home/*/.xsession-errors \ /home /mnt/BACKUP/daily.0/localhost/ /usr/bin/rsync -a --delete --numeric-ids --relative --delete-excluded \ --exclude=/home/*/.cache/ --exclude=/home/*/.local/share/Trash/ \ --exclude=/home/*/.local/share/gvfs-metadata --exclude=/home/*/.mozilla/ \ --exclude=/home/*/.cxoffice/**/Recent/ \ --exclude=/home/*/.cxoffice/**/Temp/ --exclude=/home/*/.xsession-errors \ /etc /mnt/BACKUP/daily.0/localhost/ /usr/bin/rsync -a --delete --numeric-ids --relative --delete-excluded \ --exclude=/home/*/.cache/ --exclude=/home/*/.local/share/Trash/ \ --exclude=/home/*/.local/share/gvfs-metadata --exclude=/home/*/.mozilla/ \ --exclude=/home/*/.cxoffice/**/Recent/ \ --exclude=/home/*/.cxoffice/**/Temp/ --exclude=/home/*/.xsession-errors \ /usr/local /mnt/BACKUP/daily.0/localhost/ touch /mnt/BACKUP/daily.0/ /usr/local/bin/rsnapshot_postexec.sh umount: /mnt/BACKUP: device is busy. (In some cases useful info about processes that use the device is found by lsof(8) or fuser(1)) rmdir: failed to remove `/mnt/BACKUP': Device or resource busy WARNING: cmd_postexec "/usr/local/bin/rsnapshot_postexec.sh" returned 1 rm -f /var/run/rsnapshot.pid after the fix, it works: david@silverstone:/mnt$ sudo /usr/local/bin/mount_backup_usb.sh && sudo rsnapshot -v daily echo 20199 > /var/run/rsnapshot.pid /bin/rm -rf /mnt/BACKUP/daily.6/ mv /mnt/BACKUP/daily.5/ /mnt/BACKUP/daily.6/ mv /mnt/BACKUP/daily.4/ /mnt/BACKUP/daily.5/ mv /mnt/BACKUP/daily.3/ /mnt/BACKUP/daily.4/ mv /mnt/BACKUP/daily.2/ /mnt/BACKUP/daily.3/ mv /mnt/BACKUP/daily.1/ /mnt/BACKUP/daily.2/ /bin/cp -al /mnt/BACKUP/daily.0 /mnt/BACKUP/daily.1 /usr/bin/rsync -a --delete --numeric-ids --relative --delete-excluded \ --exclude=/home/*/.cache/ --exclude=/home/*/.local/share/Trash/ \ --exclude=/home/*/.local/share/gvfs-metadata --exclude=/home/*/.mozilla/ \ --exclude=/home/*/.cxoffice/**/Recent/ \ --exclude=/home/*/.cxoffice/**/Temp/ --exclude=/home/*/.xsession-errors \ /home /mnt/BACKUP/daily.0/localhost/ /usr/bin/rsync -a --delete --numeric-ids --relative --delete-excluded \ --exclude=/home/*/.cache/ --exclude=/home/*/.local/share/Trash/ \ --exclude=/home/*/.local/share/gvfs-metadata --exclude=/home/*/.mozilla/ \ --exclude=/home/*/.cxoffice/**/Recent/ \ --exclude=/home/*/.cxoffice/**/Temp/ --exclude=/home/*/.xsession-errors \ /etc /mnt/BACKUP/daily.0/localhost/ /usr/bin/rsync -a --delete --numeric-ids --relative --delete-excluded \ --exclude=/home/*/.cache/ --exclude=/home/*/.local/share/Trash/ \ --exclude=/home/*/.local/share/gvfs-metadata --exclude=/home/*/.mozilla/ \ --exclude=/home/*/.cxoffice/**/Recent/ \ --exclude=/home/*/.cxoffice/**/Temp/ --exclude=/home/*/.xsession-errors \ /usr/local /mnt/BACKUP/daily.0/localhost/ touch /mnt/BACKUP/daily.0/ /usr/local/bin/rsnapshot_postexec.sh rm -f /var/run/rsnapshot.pid Now we have two datapoints. I can try talking to upstream? I may even still have commit rights for rsnapshot (I used to). David