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I have a hard drive partition (/dev/sdb1, ubuntu 16.04 partition)with bad sectors I intend to create an image of the partition with bad sectors and then restore to a new drive/partition.

First I run ddrescue without trying to read the bad sectors using

sudo ddrescue -n /dev/sdb1 sdb1_n.img sdb1_n_log.txt

The I run ddrescue with the -r option so that repeated attempts to read bad sectors along with the good sectors:

sudo ddrescue -d -r3 /dev/sdb1 sdb1_d_r3.img sdb1_d_r3_log.txt

Strangely the second run with -r3 gives more error locations on the log file than on the first run. The size of the images are however same.

Which image should I use for restoring the one created using -n option or the one created with -d -r3 option?

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My experience is that additional sectors are read (and their content recovered) when you run the second time with the option -r3. So use the image after running with the option -r3.

According to the tutorial chapter '9. A small tutorial with examples' in

info ddrescue

I also use -r3 (3 retries).

The number of areas marked as bad may increase, but the total amount of bad sectors will decrease, when new sectors are read during the -r3 run.

In the chapter '4. Algorithm' we find a detailed explanation,

The total error size is the sum of the sizes of all the bad-sector blocks. It increases during the trimming and scraping phases, and may decrease during the retrying phase. A sector is not marked as bad-sector and considered part of a bad area until it has been tried individually instead of as part of a large block read. Non-trimmed and non-scraped blocks are not considered bad areas. Note that as ddrescue retries the bad-sector blocks, the good data found may divide them into smaller blocks, decreasing the total error size but increasing the number of bad areas.

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