Unable to mount '/dev/sda'

My server has two 2 TB hard disk drives in soft RAID 1. I can't mount /dev/sda.

Output of parted -l

root@rescue:~# parted -l
Model: ATA HGST HUS724020AL (scsi)
Disk /dev/sda: 2000GB
Sector size (logical/physical): 512B/512B
Partition Table: gpt
Disk Flags:

Number  Start   End     Size    File system     Name     Flags
 1      20.5kB  1049kB  1029kB                  primary  bios_grub
 2      2097kB  1987GB  1987GB                  primary
 3      1987GB  1992GB  5242MB  ext4            primary
 4      1992GB  2000GB  8388MB  linux-swap(v1)  primary


Error: /dev/sdb: unrecognised disk label
Model: ATA HGST HUS724020AL (scsi)
Disk /dev/sdb: 2000GB
Sector size (logical/physical): 512B/512B
Partition Table: unknown
Disk Flags:

Output of mount /dev/sda /mnt

root@rescue:/mnt# mount /dev/sda /mnt
mount: block device /dev/sda is write-protected, mounting read-only
NTFS signature is missing.
Failed to mount '/dev/sda': Invalid argument
The device '/dev/sda' doesn't seem to have a valid NTFS.
Maybe the wrong device is used? Or the whole disk instead of a
partition (e.g. /dev/sda, not /dev/sda1)? Or the other way around?

Output of cat /proc/mdstat

root@rescue:~# cat /proc/mdstat
Personalities : [linear] [raid0] [raid1] [raid10] [raid6] [raid5] [raid4] [multipath] [faulty]
unused devices: <none>

How do I get access to my files?

Do not attempt to mount directly the device ! You need to mount a partition of it.

For example, this is wrong when you do:

mount /dev/sda /mnt

What you should do is:

mount /dev/sda3 /mnt

The system need the meta-data enclosed in the partition to know what to do with it. If you mount directly the device, these meta-data are missing and the mount will fail miserably.