Connecting a RAID set of disks to a computer may come with issues. So be warned; this is not without risk!

Keep in mind: That can be a tricky one. Did you contact QNAP support? They may have an option to assist with this.
In my experience QNAP support has been quite helpful and they may even be able to revive your TV-412 or repair it.

Having said that ... My first questions would be:

- How many disks are involved
- If RAID was used, then what kind of RAID was used

Considering it's a TS-412, then I'd guess you may be using RAID 5? (with 4 disks) ... (more info on RAID types)

Tip: if you have access to another QNAP, then it is very likely that you can just put the disks in that one and QTS (the QNAP OS) will very likely pick it up and provide access. The array may worse case show as "degraded", but it does mean you can copy your files to another disk.

Since most of us do not have a spare QNAP (that includes me), we will have to see if we can access the disks on a PC.
You mentioned a Windows PC, so that's a good starts. At least its an Intel/AMD machine, so finding a working Linux distro that offers a "live CD" (read: USB stick from which you can boot Linux) should be easy. Pick a well known one like Ubuntu (here an Askubuntu posts that shows how this could work).

We need Linux since QNAP runs a Linux variant as well, and the QNAP RAID's are typically Ext3 or Ext4 formatted, which Windows does not support (not even mentioning the RAID).

1. Can you connect all disks to your PC - does your PC have that many spare SATA connectors?
2. Once connected, you'd have to boot a Linux LiveCD (here: how to create a Ubuntu boot USB stick) or something like that, since Windows cannot read Ext3/Ext4 filesystems, not to mention that Windows may not be able to handle this particular RAID format (as far as I know).
3. Once in Linux, and disks are all connected, you'll need to use mdadm to "rebuild" or recognize the RAID. "mdadm" seems a rather common software RAID manager under Linux. See this mdadm guide.

I will admit that I have never done this, I did look on the QNAP Forum to see if anyone has documented this.
I think this thread on the QNAP forum could be very useful to get started.

If I understood this right you'll need to first "assemble" the RAID, and then mount it. I'd recommend mounting it read only (RO), to avoid damaging files of course.

As you can see ... this will take some effort, but it's not impossible!

Linux?... I don't like Linux. And on a Windows PC, there are no tools to read the RAID that is on the disks?

PS. I have RAID 5 on 4 disks.

found a video on my question ( https://youtu.be/OhzrPInWMyo?si=AiFDLpsJZ8OCL4MV ).
Are RAID recovery programs worth considering? Of course, if don`t take into account the Linux ways...

Interesting find indeed ... and it looks like their Raid recovery tool could do the trick under windows.

However, 250 Euro for a "home" license is just way too expensive. I'd even call it insane expensive. 

Consider that I just found a used TS-421 for $76 at eBay (see the listings here), or €150 in the Netherlands (this website is typically not even the cheapest).
I'm not sure where you're located, so you may find slightly different prices.
Either way: it's much cheaper than a super expensive software license, and you'd be up and running in minutes.

So no, I most definitely would not recommend buying their software.

Either get a used TS-421, which costs less than the mentioned software and you'd be up and running instantly, or go the Linux route since a "live CD" (= boot from a USB stick) is free and not much more difficult than the mentioned software (hardware setup would be the same as the software you mentioned).

Also note that their software is very likely based on open source software for some parts of at least (at the least to be compatible with mdadm).

Have you been able to determine what is wrong with your current TS-421?
Did contacted QNAP support? (they have a online ticket system, so you do not have to call them)

Posted by: @Anonymous

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Linux?... I don't like Linux. And on a Windows PC, there are no tools to read the RAID that is on the disks?

PS. I have RAID 5 on 4 disks.

It would be booting from a USB stick - so no need to install Linux. Just prepare a USB stick and boot from that.
You'd only need it for copying your files of the disks, right?

@hans understood your point of view.

So, I will try to access my data through Linux, and at the same time I will look for the same device.

Thank you.

Optionally of course; but let us known how you resolved it if you can - I'm sure others would benefit from your experience 😊