Problem noted on Ubuntu based operating systems,
This is tested with qbt 3.3 and on Ubuntu 14.04 and on Linux Mint 17.2.
Machine configuration:
one system drive (SSD) plus one internal drive, SATA 2, ext4 labelled as 'data' where the folders for qbittorrent are.
Conditions Tested:
Clean close down and restart later.
Normal reboot, client closed first
Normal reboot, client still running with all tasks paused
'force' shutdown and restart (hold machine power button)
Power failure close down (pull the 'plug' out)
Sometimes, under ANY of the restart conditions and there appears to be little or no consistency to when this will happen, the operating system decides to mount the internal disc drive with a secondary name as the mount point, while leaving the original as an empty mount point. To elaborate on that, if your disc label/mount point is "data", there will be TWO 'data' filing systems, one mounted as 'data' with no files or folders and one mounted as 'data1' where the qbt folders are.
The consequence of this is that the folders that the client is saving the torrent payloads to is gone. It is NOT that qbittorrent has 'forgotten' them, it is that they no longer exist in the the same location so all tasks go to 0% and/or have a "Missing files" status. This is NOT a 'client problem' it is an operating system problem.
The way to prevent this happening is to NEVER trust a computer after a reboot and check that ALL drives and filing systems are mounted on the same name as they were BEFORE the restart.
The 'fix' if this happens is NOT to go complaining to the developers or to start 'flapping' like a headless chicken. What you need to do is:
Pause ALL tasks and close the client
Remove the original mount point in terminal:
cd /media/your-user-name/
ls -l (to get the filing system names)
sudo rm -fr ./data (use the name of YOUR mount point 'data' is merely the example)
reboot the machine (make sure that qbittorrent is NOT going to start at boot up)
Check the mount points are correct
Restart qbittorrent and force recheck ONE TASK AT A TIME, (yes this is time consuming but if you insist on rebooting the machine that a bittorrent client runs on, this kind of thing will happen on a fairly regular basis).
Now if ANY tasks remain at 0% after the recheck you can recover them by moving ALL those payloads into the qbittorrent "Keep incomplete torrent in ..." folder, if you have your payloads sorted into folders or have 'label name folders' they will have to be ignored and only the payload IN those folders moved, once that is done then force recheck all those tasks, for this you can select all remaining tasks and run a mass recheck.
Yes, the 'cure' is painful and time consuming, so please just think ahead before just starting the client.