After almost a year in service my TVS-473 NAS suddenly dropped disk 3 and the slot refused to work since when. Sounds familiar?
Before continuing I would like to emphasize, following IS NOT approved, confirmed or authorized by QNAP! Use it with caution!
First of all don't panic your data as well as the disk itself are fine and you only need a way to backup it. To do that:
1) Power Off your NAS
2) Take the "bad" disk out
3) Connect it to the NAS backplane with male-to-female SATA data extension cable
4) Use any external power source with SATA power connector to provide power to the "bad" disk. It can be standalone PC PSU but you need to figure out how to turn it on.
5) !!!!!!!!!!!!! IT IS IMPORTANT TO MAKE ALL THE CONNECTIONS WITHOUT POWER TO NAS OR EXTERNAL SATA POWER SOURCE !!!!!!!!!!!!!!!!!!!!
6) Power on the external SATA PSU first and NAS after that
7) Wait for NAS to start, RAID to synchronize and backup all your data to external storage.
Now, when the data is safe, it is the time to contact QNAP support, and RMA your faulty NAS. It will take time and in some case money but you will get fixed NAS and keep it under the warranty.
But it is also possible to try and fix the issue by yourself. The cost - penny and 15 minutes with soldering station. !!!!!!!!!! BUT YOU WILL VOID WARRANTY AND ANY UNAUTHORIZED/UNPROFESSIONAL MODIFICATION CAN DAMAGE YOUR NAS. IT ALSO REQUIRES SOLDERING SKILLS AND KNOWLEDGE HOW TO PROPERLY HANDLE DELICATE ELECTRONIC COMPONENTS !!!!!!!!!!!!!!!!
In my opinion the only option to go is to contact QNAP. QNAP support team is helpful and professional. The issue is known to QNAP and RMA is issued on spot. And you need really good reasons to try and fix the NAS by yourself.
If you brave enough to take the second option anyway, lets start from the issue background. It looks like the devices affected are TVS-x73 manufactured before late fall of 2017. On those devices disks 3-8 SATA power is controlled by a chain of MOSFETs. High level on the Gate of control N-channel MOSFET opens high-power P-channel MOSFET and the corresponded SATA slot gets power. And it looks like the control part of the power circuit is responsible for the issue in subject. For some reason control circuit disables high-power MOSFET for a moment and it starts chain of events leading to the disk to be unplugged. Sudden loss of the power also makes HDD to produce loud "click" mentioned earlier in this thread. Another evidence is the following error in kernel log:
ata6: SError: { RecovComm PHYRdyChg 10B8B Dispar } -- Count:x
ata6: SError: { RecovComm PHYRdyChg CommWake 10B8B Dispar DevExch } -- Count:x
It is possible to modify the control circuit to lock the high-power MOSFET in open state permanently and fix the issue for good. I will not provide exact instructions what to do to avoid unnecessary casualties. But if you have right set of skills, info above is more than enough to figure out offending part and right fix procedure.
To summarize, the issue is HW related. At this stage there is no SW/FW fix. If your "Disk Unplugged", your NAS is on warranty and you see symptoms described above - contact QNAP. If it is not possible just follow the provided clues.