Disk Utility cannot verify External Hard Disc

Question

Hi,
Trying to get Disk Utility to Verify and repair an external Hard Disc, which is seen but only as 'disk1s1' not it's proper name. On plugging in it asks if I want to initialise the disk - obviously No.
DU sees the HD in the side bar but it's grey'd out, I can't mount it, Verify is greyed out.
Running diskutil in Terminal (which I'm not confident in) brings back the following - disk0 is my OS, disk1 is an install disk of Disk Warrior and disk2 is the external one I'm trying to sort out, as you can see it's got an FDisk_partition_scheme, which I think is the problem.
(BTW Disk Warrior doesn't recognise the disks either, however as this is an Apple Forum, I've not asked for advice on that here, unless someone wants to chime in on that software!)
I'm running OS 10.9.5, and it happens on multiple external disks, running in USB caddies of different manufacture, I've tried different cables, ports, restarts etc.
MBP 2.5GHz, i7, 16Gb, 500Gb SSD HD
Any advice would be most welcome as how get correct this
localhost:~ anthonyupton$ diskutil list
/dev/disk0
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *500.3 GB   disk0
   1:                        EFI EFI                     209.7 MB   disk0s1
   2:                  Apple_HFS Macintosh HD            499.4 GB   disk0s2
   3:                 Apple_Boot Recovery HD             650.0 MB   disk0s3
/dev/disk1
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:                            DW                     *67.6 MB    disk1
/dev/disk2
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:     FDisk_partition_scheme                        *3.0 TB     disk2
   1:                       0xEE                         375.1 GB   disk2s1
localhost:~ anthonyupton$ sudo gpt recover /dev/disk2
Password:
gpt recover: /dev/disk2: no primary or secondary GPT headers, can't recover
localhost:~ anthonyupton$ diskutil repairDisk /dev/disk2
Unable to repair this whole disk: A GUID Partition Table (GPT) partitioning scheme is required (-69773)
localhost:~ anthonyupton$

Answer 1

That is just a statement of optimization. It is not stating that there IS or IS NOT any errors on the disk. DiskWarrior does not actually have a separate "error detector" function. You basically let it run its disk directory data optimization routine on the volume. If it finds problems, it will attempt to fix them as it goes.
When it finishes, it will let you review its findings and results, and then you get to choose to apply the changes or not. Until you apply the changes, you are safe; if you do not apply the changes, the drive is not changed. If you apply the changes, DiskWarrior will replace the existing disk directory data with the optimized (and repaired if there was damage) version.
You should make backups of your valuable data before making repairs, in case something bad happens during your attempt to repair.
Be sure your copy of Disk Warrior is compatible with the Mac OS X release. Version 3.0.3 or later is compatible with Tiger.