Mount disk in recovery mode

Model: ATA HGST HUS726040AL (scsi)
Disk /dev/sda: 4001GB
Sector size (logical/physical): 512B/512B
Partition Table: gpt
Disk Flags:

Number Start End Size File system Name Flags
1 20.5kB 1049kB 1029kB bios_grub-sda bios_grub
2 2097kB 21.0GB 21.0GB ext4 primary raid
3 21.0GB 4000GB 3979GB ext4 primary raid
4 4000GB 4001GB 536MB linux-swap(v1) primary


Model: ATA HGST HUS726040AL (scsi)
Disk /dev/sdb: 4001GB
Sector size (logical/physical): 512B/512B
Partition Table: gpt
Disk Flags:

Number Start End Size File system Name Flags
1 20.5kB 1049kB 1029kB bios_grub-sdb bios_grub
2 2097kB 21.0GB 21.0GB ext4 primary raid
3 21.0GB 4000GB 3979GB ext4 primary raid
4 4000GB 4001GB 536MB linux-swap(v1) primary


Model: Linux Software RAID Array (md)
Disk /dev/md2: 21.0GB
Sector size (logical/physical): 512B/512B
Partition Table: loop
Disk Flags:

Number Start End Size File system Flags
1 0.00B 21.0GB 21.0GB ext4


Model: Linux Software RAID Array (md)
Disk /dev/md3: 3979GB
Sector size (logical/physical): 512B/512B
Partition Table: loop
Disk Flags:

Number Start End Size File system Flags
1 0.00B 3979GB 3979GB ext

this is my disk
why with this command mount /dev/mdX /mnt/root
don't work
thanks