$ sudo mdadm --create /dev/md0 --level=1 --raid-devices=2 /dev/nvme0n1p1 --write-mostly /dev/sdc1 mdadm: Note: this array has metadata at the start and may not be suitable as a boot device. If you plan to store '/boot' on this device please ensure that your boot-loader understands md/v1.x metadata, or use --metadata=0.90 mdadm: /dev/sdc1 appears to be part of a raid array: level=raid1 devices=2 ctime=Sat Jan 9 15:22:29 2021 Continue creating array? y mdadm: Defaulting to version 1.2 metadata mdadm: array /dev/md0 started. @KE: @KT:Listing 9: View the RAID Volume @LI:§§nonumber $ sudo mdadm --detail /dev/md0 /dev/md0: Version : 1.2 Creation Time : Sat Jan 9 15:52:00 2021 Raid Level : raid1 Array Size : 244065408 (232.76 GiB 249.92 GB) Used Dev Size : 244065408 (232.76 GiB 249.92 GB) Raid Devices : 2 Total Devices : 2 Persistence : Superblock is persistent Intent Bitmap : Internal Update Time : Sat Jan 9 15:52:21 2021 State : clean, resyncing Active Devices : 2 Working Devices : 2 Failed Devices : 0 Spare Devices : 0 Consistency Policy : bitmap Resync Status : 1% complete Name : dev-machine:0 (local to host dev-machine) UUID : 833033c5:cd9b78de:992202ee:cb1bf77f Events : 4 Number Major Minor RaidDevice State 0 259 2 0 active sync /dev/nvme0n1p1 1 8 33 1 active sync writemostly /dev/sdc1