i have a hdd that crashed and i sent it to commercial recovery. The recovery team has the encryption key to decrypt it.

the feedback i got was
We are currently still trying to recover your data, head 0 is giving us a bit of a headache.

Tech report:

Transplanted heads. Made SA backup. 1x NTFS partition, it is encrypted.
Transplanted heads 3 times, but head 0 dies after a while.
Mirror finished with 2.3M sectors not read (head0).

What does this mean? is the drive busted?

2.3 million sectors unread is not a huge number on most modern drives. A 1Tb drive has around 2 Billion sectors total. So it sounds like they recovered a majority of the data. I'd ask them for a listing of the files with bad/unread sectors.

Unless of course those bad/unread sectors contain the disk encryption key, in which case you are definitely screwed.

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Data Medics - Hard Drive, SSD, and RAID Data Recovery Service Company

Drive stores your data on magnetic platters, where the smallest portion of data that drive operates is a sector (4096 bits). Modern drives consist of a millions and billions sectors. So, in your case you've lost some part of disk (data). Actually, 2,3M sectors it is not too much, if there was a serious mechanical problem. It is better if all unsuccessful sectors are located in one region, in this case more data will survive.

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http://www.alfadatarecovery.com