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.

Based on Kommisar's post we gave it a shot today and it looks to be working. Disk 3 is on, and raid is rebuilding. If you are going to attempt this on your own boards, read and understand Kommisar's post so you can trace your own board. Verify your problem is identical before trying this solution. Here is what was needed for my specific scenario (disk 3 failure, eventually leading to no power).

For reference, my board is labelled TS-473 BP V1.4 (QY04): The mosfet that seems to be the issue is on the opposite side, and is labelled 4957AGM. Google for pinouts.
That IC contains 2 mosfets. Tracing the pins looks like one feeds the 12V sata pins and the other feeds the 5V sata pins (at least for bay 3, other bays may need a different fix). Shorting the source and drain of each will bypass the mosfet and supply power permanently to the sata ports. One or both could be faulty, in my case it looks like only the 5V fix was needed but we did both. You could also just replace the IC with another 4957AGM, although it may still fail again (something caused it to fail originally).

Shorted: Make these changes at your own risk. Modifying a circuit should be your last resort! RMA it if you are not familiar with what is shown here (or don't have a friend who is!).

Might not hurt to ask QNAP support if they will offer up a replacement board for sale (it's an easy enough swap), assuming they have a solution to the fault.

Thanks Kommisar for the diagnostic work!

Shmal, kudos for finishing the quest! Indeed, the simplest, robust and most cost-effective solution is to provide permanent power to the slot by removing the power MOSFET and by soldering jumper wires instead. There is also a number of other options like lo remove control MOSFET and replace it with a resistor or trying to replace the power MOSFET and hope the new one will work longer. But as I mentioned it earlier, simple jumper wire works great in this case and it is really bulletproof.

In generall this NAS is really good and pity it has this issue. I believe most of the devices are out of warranty now and it is safe to share this fix because it will help some people to save a lot of money and avoid unnecessary frustration.

!!!!!!!!!! BUT AGAIN, 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 !!!!!!!!!!!!!!!!

Hi,

I'm french and will make an effort to be understandable.

I have read the Komissar's post and now I have my TS-431 fully functional.
Here the fix for this model.

My problem was on the 4th bay with the "Disk unplugged" error several times by minute.

To confirm my problem, I have powered the 4th disk with external PSU like this : With this mounting, all is OK. So it's a power default like explained by Komissar.

My back plane with the MOSFET of the third and fourth bay in red circle: The MOSFET is a NTMD6P02 : https://www.onsemi.com/pub/Collateral/NTMD6P02R2-D.PDF

With a multimeter, the document above and a documentation about SATA power, my conclusion is :
- D1 S1 G1 is for the 12V
- D2 S2 G2 is for the 5v

Si, I maked short circuits between D1 and S1, S2 and S2 to inhibt the MOSFET.
Trick : there is two drain (D1 and D1) and (D2 and D2). So, you can choose any drain or overflow with tin between D1 and D1 or D2 and D2 it's not a problem.
My soldering iron is not thight so my welds are not very ** but functional The backplane in place : Thanks for the trick. My NAS was shutdown for 2 years because of this default.

I'm open to questions about this model if necessary.

Thanks !

Sylvain (from France)