Erasing individual disk from a RAID

[northernbaldy]

This is driving me mad now

My ReadyNAS has crapped out so I tried to factory reset the thing
It fails saying it can't reformat the 4 installed disks

I took all the disks out and tested them in a caddy to see which, if any, of the disks had died. They all seem to be fine.
So my next thought was to erase them, put them back in the NAS and set it up as if they were new disks

This is where I'm getting stuck
Disk utility says it can't erase the drive because there isn't enough space.
I'm guessing it thinks the individual 2TB disks are 4TB (as they were in the raid)

What can I do to erase them?

 

[Taz Mangus]

Have you tried contacting NetGear for assistance?

 

[northernbaldy]

Dug out the trusty old 17"

sorted
I'm not sure why apple have dumbed down disk utility, it was nice and simple to start with

apple, it just works (well, they used to)