I finally managed to fix my WD MyCloud !!
Here are the steps I got through:
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first I opened up the box, got the Hard Drive out and plugged it on my laptop (SATA to USB)
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then I tried to restore the rootfs.img file from the latest firmware onto the two RAID partitions, but it didn’t work
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so I tried to restore the entire disk image with those findable on these pages: http://community.wd.com/t5/WD-My-Cloud/Here-is-WD-My-Cloud-2TB-virgin-disk-image/td-p/637769
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BUT IT FAILED AGAIN (I’m afraid compression/upload and download/decompression might have corrupted the image)
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so I bought another WDMC, upgraded it to the latest firmware, plugged another disk on it, then backup all partitions on this disk except the 4th (the extended 1.9TB one) through SSH, with that command: dd if=/dev/sda1 of=/shares/YOURDISK/partition1 (repeat for /dev/sda2, 3, 5, 6, 7 and 8)
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at this time I was able to restore the seven images one by one, using Disks utility on Linux (dd would also have worked I guess), AND IT WORKED
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I just had to do a Quick Restore because the web interface said 0Ko free space (Quick Restore is always good after debrick)
So, to summarize, I
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downloaded the entire disk image (link above)
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wrote it on the WDMC disk
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used clean WDMC partitions (all but the big one) to restore those on the WDMC disk
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(did a Quick Restore)
If anyone needs them, I can upload them on something like Mega or Uptobox. The problem is that it is about 5Go, so it would take some time. I can compress them using 7zip but again, I fear data corruption by doing so.
Anyway, many thanks to the community 
!!
