Hi !
Shall I diagnose with hddsupertool 2Tb WDC2003FYYYS devices ?

It returns no SMART info: shall I consider it as dead ?

it is not a diagnostic tool, it is a script to apply slow-fix on western digital drives having this problem.

what is the problem with your disk ?
provide details so someone can point you in right direction.

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Data Recovery Pakistan

You might want to refer to my post, (I have slow WD drive) I also had problems getting hddsupertool to dump files and show SMART info until I switched to another SATA controller that used a JMicron chip.

http://forum.hddguru.com/viewtopic.php?f=1&t=32987

As for diagnosing, hopefully another member can point you in the right direction as I'm also not sure.

My device took part of a RAID5 array composed of 4-2Tb-WD2003FYYS devices. Suddenly two devices failed.
I rebooted the machine (with the 4 devices) and was in grub-rescue mode. Only 2 devices were visible.
I rebooted the machine and went in the BIOS. The two failed devices were invisible. I hear the spin up 9 times.
The smart data cannot be read. All commands, with Debian, using /dev/sdxy don't work.

For me the disk was dead and I discovered the HDDsupertool application.

Only this application can see the failed devices with the hddsupertool -d command.
I see ata1 - cff0 cf7e cfa0 1 wdc2003FYYS-02W0B1

What could HDDsupertool bring me ?
I am aware that I lost 1/3 of data and I would like to retrieve slices of data.

Without getting a better sense of drive behavior no tool can bring anything.
Have to understand problem first, then think and implement solution.
I would suggest testing the drive with something like MHDD. Determine status register on drive and whether it IDs with correct name and capacity. Also, listen to its physical behavior closely and write down what it does exactly. Spin, no spin, spin and spin down after whatever seconds, etc.

P.S. You are misunderstanding data loss in RAIDs. It does not quite work like that, meaning one drive loss, then 1/3 data loss. There are a few very important variables that have a lot of weight in whether you can get back any files. Either way, if the data is important, then it is strongly recommended to see a specialist. It is a tough learning curve to pull a successful recovery out of this one on your own.

Ensure not mixing up drive order if you continue with testing.

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Hard Disk Drive (HDD), Solid State Drive (SSD, SATA, NVMe, etc), USB Flash Drive and RAID Data Recovery Specialist in Massachusetts