If your QNAP for some reason has a dirty RAID volume and you cannot run the disk check from the UI, because it cannot unmount the volume, drop down to SSH and try this instead:
/etc/init.d/services.sh stop /etc/init.d/opentftp.sh stop /etc/init.d/Qthttpd.sh stop umount /dev/mapper/cachedev1 e2fsck_64 -f -v -C 0 /dev/mapper/cachedev1 reboot
Stopping all the services takes some time, just be patient… and then
you’ll be able to unmount,
e2fsck and reboot your NAS to get it up
and running again.