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I have a 3TB WD Green HDD that failed. I bought two WD Red 3TB HDD + Synology DS215j NAS. Before installing NAS in RAID1, I want to use the WD Reds to recover the data from the 3TB WD Green HDD (one WD Red for the image file, then the other WD Red to recover data from image; when finished, I want to format the WD Red with the image file and insert it into the NAS case, then copying into it the recovered data, then add the second WD Red to use the NAS in RAID1 for redundancy).

I partitioned one of the two WD Red with ext3 filesystem and started imaging. After four days, the image reached the 2TB file size limit for ext3 and I was not able to resume to another image file on the same disk.

  • Question 1: Even if I use the second WD Red, formatting it with ext4 and copying into it the 2TB image file, I am not sure if I will be able to complete the imaging, because the failed drive and the WD Red have both the same size (3TB). I think I need a second destination drive of at least a few GB (I have a 500 GB one). What do you think?

  • Question 2: Why I am not able to resume the imaging to a second file in the same disk which contains the 2TB image file? (ddrescue says again write failed, file too large). I read a lot of forum posts between yesterday evening and this morning, but I only found guides on how to mount two different image files, what is missing is how to resume a ddrescue operation with a different image file (they say it is simple, since but in my case it does not work and I need to discover why).

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If you want to rescue the whole partition, maybe it had been a better idea if you did the target a whole hard disk, not an individual file. You could have gave simply the target disk as outfile, so:

ddrescue /dev/baddisk /dev/gooddisk

It is not a problem if your gooddisk is bigger, after the rescuing you could solve that problem by playing with the partition table.

Another solution: instead of ext3, use ext4. In ext4, the maximal file size grown to 16TB from the 2TB of the ext3. You can convert an ext3 to ext4 very easily, actually the most easier way: simply mount it as an ext4 partition.

Extension: Maybe it won't be enough. Here is a doc, how you can migrate ext3 to ext4. It is relatively easy.

  • Hi, converting ext3 to ext4 simply mounting did not work, same error (write error, file too large). I am redoing the whole process again, using the whole disk as destination and a usbkey to store the logfile. When the disk clone is complete, I have to run ddrescue again or simply use chkdsk (Windows) / another check disk tool in linux? By the way, if in future I need to split output into two image, what is the correct command sequence of ddrescue? I tried something like (leads to "write error file too large" message) sudo ddrescue -r 3 /dev/sdc /mnt/image2ndpart /mnt/samelogfile – Davide Norbiato Sep 2 '15 at 8:02
  • @DavideNorbiato Well, maybe you have right. Here is a doc, how you can migrate ext3 to ext4. It is relatively easy. – peterh Sep 4 '15 at 19:20

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