WD Blue 6 TB (WD60EZRZ), slowly failing. Read errors are getting worse with each read, likely a weak head. Partial clone done, but critical files are missing. Considering Blizzard Data Recovery or Rossmann Group. Any experience with either for head swaps?

Both will be fine - If it's still reading it may not even need replacing. Rossmann has a longer turn around time if I recall , especially at the moment. Blizzard is also likely to be cheaper if that's a concern.

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

I suppose it's a such a common operation, it's a bit like asking which garage does the best tire change. It's kind of a nonsense question.

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Joep - http://www.disktuna.com - video & photo repair & recovery service

I found WD blue hard disks to be rather flakey at best. Some Seagate barracuda disks are equally dismal

I turned to enterprise class disks which have lasted

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USB 3.5" enclosures are $19 USD postage paid from China, I own several

Which enterprise disks?

what is the definition of "flakey"?

How long did your enterprise disks last?

did you run Seatools on them? as it would have fixed them