Recovering data from a failed RAID array with ddrescue: a practical walkthrough

When a RAID array fails, the worst thing you can do is panic and start poking at it immediately. I've seen too many cases where an impatient rebuild attempt overwrote the only good copy of data. This walkthrough covers how to safely approach a degraded or failed RAID — with ddrescue as your best friend. Step 0: Stop. Don't touch the array yet. Before running mdadm --assemble , before doing anything, clone your physical disks . A RAID 5 with one failed drive can lose everything the moment a second drive throws a read error during rebuild. This isn't hypothetical — it's how most total RAID losses happen. The golden rule: image first, recover second . Step 1: Assess the damage # Check current RAID state cat /proc/mdstat # More detail mdadm --detail /dev/md0 Look for: [UUU_] — one drive failed (underscore = missing) [UU__] — two drives failed (catastrophic for RAID 5) State: degraded , recovering , or failed Do NOT run mdadm --manage /dev/md0 --add /dev/sdX yet. Stop the array instead: mdadm --stop /dev/md0 Step 2: Clone each disk with ddrescue ddrescue is the right tool because it handles read errors gracefully: it maps bad sectors, retries them, and lets you resume interrupted sessions. Never use dd for a failing disk. Install it: # Debian/Ubuntu sudo apt install gddrescue # RHEL/CentOS sudo dnf install ddrescue Clone each RAID member to a separate image file (you need enough storage — same total size as all disks combined): # First pass: copy everything readable, skip bad sectors fast sudo ddrescue -d -r0 /dev/sda /mnt/backup/sda.img /mnt/backup/sda.log # Second pass: retry bad sectors up to 3 times sudo ddrescue -d -r3 /dev/sda /mnt/backup/sda.img /mnt/backup/sda.log Key flags: -d — direct disk access (bypass kernel cache) -r0 / -r3 — retry bad sectors 0 or 3 times The .log mapfile is critical: it lets you resume if the clone is interrupted Repeat for every disk in the array ( sdb , sdc , etc.). Step 3: Work from the images Once you have image files, assemble a soft