In other words, what not to buy if there's a possibility you may need data recovery in the future.

Helium filled for starters.

Now it gets messy:

Seems like WD is problematic with some Charger and Passport models as well as other models which are hard to identify just by part number.

And Hitachi's too.

I wonder if hardware encryption on a HDD is a problem? and I mean the presence of the hardware, but not using it to encrypt the drive.

Does this leave Seagate with a "clean bill of health?"

I have no idea so I'm asking....

Anything mechanical can and will fail at some point, if you're concerned about the difficulty of future data recovery ensure you never need to do it by having and using an adequate back up policy and recovery plan. If you're following a decent 3-2-1 strategy the medium storing the current active data becomes almost irrelevant. Hard drives are cheap, data recovery is not.

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

Yes, but accidents happen. Why not buy stuff that is recovery friendly?

I've just learned that many non-Helium HDD's do not have removable covers!

How does one avoid them?

Toshiba for HDD
Samsung 840 for SSD