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Never try to rescue a r/w mounted partition. The resulting copy may be useless. source: http://www.gnu.org/software/ddrescue/manual/ddrescue_manual.html

what does this mean exactly?

I have finally got my drive to the point where it is reading from my disk consistently, but I can see the partition in question from "computer" in Ubuntu. Does this mean that my new backup copy is toast and I need to start over?

Why exactly could the resulting copy be useless?

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up vote 3 down vote accepted

Copying an image from a disk takes time. If the disk is mounted and in use at the time there will be changes made to the disk while the image is being made.

These changes will be inconsistant and may cause filesystem corruption.

Depending on which filesystems are on the disk they may be able to recover from this corruption, or it may cripple the filesystem completely.

Any filesystems in the Computer menu have been detected, but not mounted unless you specifically click on it (and a window opens) in the menu, or unless there is a specific instruction to mount it in somewhere like /etc/fstab.

The easiest way to tell is to open a Terminal window and type "mount"

That will list all the mounted filesystems. If your disk is listed in there then there is a good chance that there will be some filesystem corruption in your image.

If it's not listed then your image should be clean.

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two reasons - firstly, it would not result in a consistent backup, since files could have changed in the middle of a backup (resulting in a backup that's not the same as the original)

. Secondly, in the case of DDrescue, you normally want to use it as a last ditch backup method for disks you suspect are damaged - you want to minimise read/write cycles other than the backup cycle.

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