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.