# Replace a failed disk in Linux software RAID (mdadm) > A disk in your server's mdadm RAID failed? Confirm it, note the serial, remove it from the array, have it replaced, then rebuild the mirror and the boot loader. Difficulty: Advanced Most dedicated servers with two disks run **Linux software RAID 1**: both disks hold the same data. When one fails, the server keeps working on the other, and the array is **degraded**. This guide takes you from the warning to a healthy mirror again. For the day-one checks of your RAID and disks, see [dedicated servers](/guides/dedicated-servers). ## Before you start - Root access over SSH, and the server's IPMI console if your server has one ([IPMI console and virtual media](/guides/ipmi-virtual-console-and-media)). - **A current backup.** During a degraded state and the rebuild, your data exists only once; see [backup strategy](/guides/backup-strategy-3-2-1). - In the examples the healthy disk is `/dev/sda`, the failed one `/dev/sdb`, and the arrays are `/dev/md0` (partitions 1) and `/dev/md1` (partitions 2). Use the names from your own output. ## Step 1: Confirm which disk failed ```bash cat /proc/mdstat sudo mdadm --detail /dev/md0 ``` In `/proc/mdstat`, a healthy RAID 1 shows `[UU]`; a degraded one shows `[U_]` or `[_U]`, and a failed member is marked `(F)`. `mdadm --detail` shows `State : clean, degraded` and lists the faulty or removed device. Note the **serial number** of the failed disk, so the right disk is replaced: ```bash lsblk -o NAME,SIZE,MODEL,SERIAL sudo smartctl -i /dev/sdb ``` If the failed disk no longer answers, note the serial of the **healthy** disk instead and say so in the ticket. ## Step 2: Remove the failed disk from every array A disk usually has a partition in each array. Mark each one as failed and remove it: ```bash sudo mdadm /dev/md0 --fail /dev/sdb1 --remove /dev/sdb1 sudo mdadm /dev/md1 --fail /dev/sdb2 --remove /dev/sdb2 ``` **Verify:** `cat /proc/mdstat` lists only `sda` partitions in each array. ## Step 3: Ask us to replace the disk Open a ticket under **Support › Tickets › Open Ticket**, choose the dedicated server as **Related Service**, and write: - that a RAID member failed and has been removed from the arrays; - the **serial number** (and model) of the failed disk, or of the healthy disk to keep; - the output of `cat /proc/mdstat`; - when a replacement suits you. ![The Open Ticket form with the subject Disk replacement: failed RAID member in server1.example.com, the Technical Support department, the dedicated server as Related Service and a message with the failed disk's serial number](https://hyperdc.com/templates/hyperdc/img/docs/replace-failed-raid-disk/ticket.en.webp "A disk replacement request. Server name and serial number are examples.") We confirm the time and the procedure with you; see [open a support ticket](/guides/open-a-support-ticket). ## Step 4: Copy the partition layout to the new disk After the replacement, find the new, empty disk with `lsblk` (no partitions, same size or larger). Check the partition table type of the healthy disk: ```bash sudo fdisk -l /dev/sda | grep -i "disklabel type" ``` > **Warning** > > The next commands overwrite the partition table of the **target** disk. Double-check that `/dev/sdb` is the new, empty disk. **GPT** Copy the table from `sda` to `sdb`, then give the copy its own unique GUIDs: ```bash sudo sgdisk -R /dev/sdb /dev/sda sudo sgdisk -G /dev/sdb ``` **MBR (dos)** ```bash sudo sfdisk -d /dev/sda | sudo sfdisk /dev/sdb ``` **Verify:** `lsblk` shows the same partitions on `sdb` as on `sda`. ## Step 5: Add the new partitions and let the arrays rebuild ```bash sudo mdadm /dev/md0 --add /dev/sdb1 sudo mdadm /dev/md1 --add /dev/sdb2 watch cat /proc/mdstat ``` `/proc/mdstat` shows `recovery` with a progress bar and an estimated finish. Leave `watch` with `Ctrl` + `C`; the rebuild continues in the background. **Verify:** when the rebuild is done, every array shows `[UU]` and `mdadm --detail` reports `State : clean`. ## Step 6: Make the new disk bootable The boot loader lives outside the arrays, so put it on the new disk too: **BIOS (legacy) boot** ```bash sudo grub-install /dev/sdb ``` **UEFI boot** If `/sys/firmware/efi` exists, the server boots with UEFI. The EFI system partition is not part of the RAID; copy it from the healthy disk to the empty partition created in Step 4 (here partition 1 is the EFI partition; check with `lsblk -o NAME,PARTTYPENAME`): ```bash sudo dd if=/dev/sda1 of=/dev/sdb1 bs=1M ``` ## Troubleshooting **`mdadm: cannot find /dev/sdb1: No such file or directory` when removing.** The failed disk has vanished from the system. Remove the missing member with `sudo mdadm /dev/md0 --remove detached` (or `failed`), then continue. **The rebuild is very slow.** Rebuilds share the disks with normal traffic. Avoid heavy disk work until it finishes. **The server does not boot after the replacement.** The boot loader is missing on the disk the firmware tries first. Use the IPMI console to choose the other disk, then complete Step 6. **A second disk reports errors during the rebuild.** Stop and open a ticket immediately with `cat /proc/mdstat`, `mdadm --detail` and `dmesg` output; do not force anything before your data is safe. ## Next steps - Check disk health regularly with `smartctl -H` on each disk; see [dedicated servers](/guides/dedicated-servers). - Keep restores possible: [backup strategy](/guides/backup-strategy-3-2-1). ## Frequently asked questions ### Is my data lost when one disk of a RAID 1 fails? No. A RAID 1 mirror keeps a full copy on the other disk, so the server keeps running. Until the rebuild is finished, though, there is no second copy: make sure your backups are current. ### Can the disk be replaced while the server runs? That depends on the server's hardware. Tell us the serial number of the failed disk in your ticket and we confirm the time of the replacement with you, including whether the server must be switched off. ### How long does a rebuild take? It depends on the disk size and how busy the server is. /proc/mdstat shows the progress and an estimated finish time. ### Does this apply to hardware RAID controllers? No. This guide covers Linux software RAID managed with mdadm. Hardware RAID controllers have their own tools; open a ticket with the controller's status output. --- Source: \ Updated: 2026-10-15