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