How to run /sbin/fsck -fy on the recovery partition if filesystem is unknown Disk utility says that there is an unknown filesystem on my harddisk. The main (and only OS) partition seems okay, though. 

I also checked this via /sbin/fsck -fy in terminal.

Unfortunately, this command just checks /dev/disk0s2, which is the main partition. I would like to check disk0s3 since I suspect my recovery partition to be broken.

$ diskutil list
/dev/disk0
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *4.0 TB     disk0
   1:                        EFI EFI                     209.7 MB   disk0s1
   2:                  Apple_HFS iMac HD                 4.0 TB     disk0s2
   3:                 Apple_Boot                         577.3 MB   disk0s3
I take no responsibility for what happens to your mac. The command line can destroy your working computer. Fsk can fix things better than disk utility though.
If you agree, open terminal on your the mac.

To repair disks first find their disk identifiers. They are all different.
In the terminal type without quotes "diskutil list"

In the left-hand column, you will see all identifiers for the drives unmounted and mounted. it will look like this disk2b1, put this in the the diskabc position below.

In the "sudo /sbin/fsck -fy /dev/diskABC"

Type your password for best effect! You will not see the characters of the password so type well.