I needed to mount a Linux software RAID partition directly to copy the data from it; it wasn’t being recognised as a RAID device (it didn’t appear in /proc/mdstat, only my newly-created array did), and attempting to mount it gave me:

[davidp@supernova:~]$ sudo mount /dev/sdc4 /mnt/tmp
mount: unknown filesystem type 'mdraid'

To get round that, I had to tell mount the filesystem type that was actually in use:

sudo mount -t xfs -o ro /dev/sdc4 /mnt/tmp

(I mounted it read-only just in case this approach wasn’t going to work; I don’t want to write to it anyway.)

Might be useful for anyone Googling (it’s pretty obvious, but it made me think for a few minutes… it’s getting late!)

The partition type appeared as ‘Linux raid autodetect’:

Disk /dev/sdc: 200.0 GB, 200049647616 bytes
255 heads, 63 sectors/track, 24321 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Disk identifier: 0x7bf29ced

   Device Boot      Start         End      Blocks   Id  System
/dev/sdc1   *           1         871     6996276   fd  Linux raid autodetect
/dev/sdc2             872         995      996030   fd  Linux raid autodetect
/dev/sdc3             996        1119      996030   82  Linux swap / Solaris
/dev/sdc4            1120       24321   186370065   fd  Linux raid autodetect