I have got a Toshiba MQ04ABF100 disk . It is getting detected ( although it takes a quite long time) - File Explorer adds the disk as a new driver -
but it stays busy and no files are listed (in File Explorer in Windows.)
Where can be a problem? Is it possible to save the data?
Thank you for suggestions.

Try cloning it asap before it dies - these drives are no fun to work with once they start acting up, hddsuperclone or ddrescue.

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Data Recovery Services in the UK.
https://www.usbrecovery.co.uk/

Thank you for your reply.
Do you think I will be able to clone the disk even in the current state?
And will the clone work in a correct way?

You may get lucky besides your only other option is to send it to a lab for recovery. As to whether a clone will work, that entirely depends on how much can be cloned from the drive - the hardware is obviously failing how much you can get before it causes the firmware to fail is anyone's guess without professional tools.

_________________
Data Recovery Services in the UK.
https://www.usbrecovery.co.uk/

Is this model supported by PC3K or MRT?

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A backup a day keeps DR away.

Is it a software problem or a hardware problem? How can I find that?
The customer did not use PC3K nor MRT

Not properly IIRC still the same issues as discussed before and I don't think that's changed with the last update.
IMO it's hardware displaying as logical and will result in firmware. End users aren't expect to have or use PC3K or MRT.

_________________
Data Recovery Services in the UK.
https://www.usbrecovery.co.uk/

In fact it is not.
Yeah, it IDs and there is a family choice for them, but except from the basic stuff, the only useful thing you can do is Techno Off. And sending VSCs through the ATA Commander, of course.

I believe it will be supported much better in the following versions.

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