OpenWRTのRAID-1(ミラーリング)で運用していたディスクを取り外し、1台だけ別のシステム(具体的にはRaspberry Pi)に接続してマウントする。
一般的なLinuxシステムでも同じ方法でできるはず。

mdadmをインストール

sudo apt install mdadm

取り外したHDDを適当なHDDケースに入れて、RaspberryPiにUSB接続する。

そのときのdmesgの抜粋

[ 6150.566933] usb 2-2: new SuperSpeed USB device number 2 using xhci_hcd
[ 6150.588314] usb 2-2: New USB device found, idVendor=152d, idProduct=0578, bcdDevice=72.05
[ 6150.588337] usb 2-2: New USB device strings: Mfr=1, Product=2, SerialNumber=3
[ 6150.588349] usb 2-2: Product: GW3.5AM-SU3P
[ 6150.588359] usb 2-2: Manufacturer: Kuroutoshikou
[ 6150.588368] usb 2-2: SerialNumber: 0C51800004CB
[ 6150.630844] scsi host0: uas
[ 6150.632117] scsi 0:0:0:0: Direct-Access     ST3000DM 007-1WY10G       7205 PQ: 0 ANSI: 6
[ 6150.706932] sd 0:0:0:0: [sda] 5860533168 512-byte logical blocks: (3.00 TB/2.73 TiB)
[ 6150.706952] sd 0:0:0:0: [sda] 4096-byte physical blocks
[ 6150.707175] sd 0:0:0:0: [sda] Write Protect is off
[ 6150.707182] sd 0:0:0:0: [sda] Mode Sense: 53 00 00 08
[ 6150.707533] sd 0:0:0:0: [sda] Disabling FUA
[ 6150.707540] sd 0:0:0:0: [sda] Write cache: enabled, read cache: enabled, doesn't support DPO or FUA
[ 6150.741115] sd 0:0:0:0: Attached scsi generic sg0 type 0
[ 6150.763114] sd 0:0:0:0: [sda] Preferred minimum I/O size 4096 bytes
[ 6150.763130] sd 0:0:0:0: [sda] Optimal transfer size 33553920 bytes not a multiple of preferred minimum block size (4096 bytes)
[ 6150.811705]  sda: sda1 sda2 sda3
[ 6150.812083] sd 0:0:0:0: [sda] Attached SCSI disk
[ 6150.922693] block device autoloading is deprecated and will be removed.
[ 6150.938755] block device autoloading is deprecated and will be removed.
[ 6150.971161] block device autoloading is deprecated and will be removed.
[ 6150.994442] md/raid1:md125: active with 1 out of 2 mirrors
[ 6151.066078] md125: detected capacity change from 0 to 5850827520
hiroshi@omvpi:~ $ 
hiroshi@omvpi:~ $ 

hiroshi@omvpi:~ $ sudo fdisk -l /dev/sda
Disk /dev/sda: 2.73 TiB, 3000592982016 bytes, 5860533168 sectors
Disk model: 007-1WY10G      
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disklabel type: gpt
Disk identifier: 36F7B8D6-ED67-4C9C-A0A9-51CD930408B4

Device       Start        End    Sectors  Size Type
/dev/sda1       64    8388671    8388608    4G Linux RAID
/dev/sda2  8388672    9437247    1048576  512M Linux RAID
/dev/sda3  9437248 5860529072 5851091825  2.7T Linux RAID
hiroshi@omvpi:~ $ 

hiroshi@omvpi:~ $ ls -l /dev/md*
brw-rw---- 1 root disk 9, 125 Jan 26 21:19 /dev/md125
brw-rw---- 1 root disk 9, 126 Jan 26 21:19 /dev/md126
brw-rw---- 1 root disk 9, 127 Jan 26 21:19 /dev/md127

/dev/md:
total 0
lrwxrwxrwx 1 root root 8 Jan 26 21:19 OpenWrt:3 -> ../md125
hiroshi@omvpi:~ $ 
hiroshi@omvpi:~ $ cat /proc/mdstat 
Personalities : [raid1] 
md125 : active (auto-read-only) raid1 sda3[0]
      2925413760 blocks super 1.2 [2/1] [U_]
      bitmap: 3/22 pages [12KB], 65536KB chunk

md126 : inactive sda2[0](S)
      522240 blocks super 1.2
       
md127 : inactive sda1[0](S)
      4189184 blocks super 1.2
       
unused devices: <none>
hiroshi@omvpi:~ $ 

マウントすることで、中身にアクセスできた。
（リードオンリーのようだけど、データを救い出せることが分かったので目的は達せられた）

hiroshi@omvpi:~ $ sudo mkdir /mnt/hdd{1,2,3}
hiroshi@omvpi:~ $ sudo mount /dev/md125 /mnt/hdd3
hiroshi@omvpi:~ $ ls -l /mnt/hdd3
total 32
drwxr-xr-x 3 root   root  4096 May 28  2022 homes
drwx------ 2 root   root 16384 May 28  2022 lost+found
drwxrwxrwx 8 nobody root  4096 Oct 15  2022 share
-rw-r--r-- 1 root   root    29 May 28  2022 TESTFILE
drwxr-xr-x 6 nobody root  4096 Nov 23  2022 users
hiroshi@omvpi:~ $ 

有効になっていないパーティションも読めるようにする。