data recovery from a disappeared partition

mrmazda · 04-03-2022, 01:00 PM

Quote:

Originally Posted by ondoho

dd the complete drive to a file on a healthy drive with sufficient space, and work on that copy, NOT the physical drive itself.

ddrescue should create a more complete file.

beachboy2 · 04-03-2022, 02:18 PM

mrokii,

One link for using ddrecue to clone a hard drive:

https://datarecovery.com/rd/how-to-c...with-ddrescue/

ondoho · 04-04-2022, 11:59 AM

Quote:

Originally Posted by mrmazda

ddrescue should create a more complete file.

How can it be more complete than dd'ing the whole drive?

boughtonp · 04-04-2022, 12:29 PM

Quote:

Originally Posted by ondoho

How can it be more complete than dd'ing the whole drive?

Apparently it has logic to skip bad/erroring parts, so it can potentially get more data before a drive fails completely.

https://www.gnu.org/software/ddrescue/

Quote:

Originally Posted by https://www.gnu.org/software/ddrescue/manual/ddrescue_manual.html#Algorithm

GNU ddrescue is not a derivative of dd, nor is related to dd in any way except in that both can be used for copying data from one device to another. The key difference is that ddrescue uses a sophisticated algorithm to copy data from failing drives causing them as little additional damage as possible.

...

Ddrescue manages efficiently the status of the rescue in progress and tries to rescue the good parts first, scheduling reads inside bad (or slow) areas for later. This maximizes the amount of data that can be finally recovered from a failing drive.

The standard dd utility can be used to save data from a failing drive, but it reads the data sequentially, which may wear out the drive without rescuing anything if the errors are at the beginning of the drive.

Shame that explanation isn't more up-front/explicit in the man page.

mrmazda · 04-04-2022, 12:40 PM

Quote:

Originally Posted by ondoho

How can it be more complete than dd'ing the whole drive?

Have you no familiarity with ddrescue? dd retries are limited (non-existent?). ddrescue retries are configurable, IIRC up to unlimited. It compares multiple tries before deciding whether a sector has been successfully read. Difficult reads are postponed until after first pass is complete, then going back to try again later, in order to preserve whatever life may remain. ddrescue also (optionally) logs and provides continually updated status.

ondoho · 04-04-2022, 01:02 PM

Thanks both. I'd have phrased it differently, but I see what you mean now. I rarely clone disks so no, I'm not familiar with it.