I have an external HD with 3 partitions, one as a backup and 2 others. I have tried to merge the 2 other partitions using a RAID Concatenated Disc in Disc Utility but it won't let me add any discs to the list.

I am trying to get around wiping the whole drive because backing up my internal drive takes several hours from scratch and I don't want to buy special partition software (like iPartition) for a single task.

Am I missing something? Does it matter what the partition scheme is on the ext HD? (it says its Master Boot Record)

I do not believe you can RAID partitions. Software RAID via Disk Utility will destroy what is stored on the disk as well.

Probably not supported for volumes on the same disk... seems utterly pointless to me

Thanks for the replies. I repartitioned and will start on my full backup again.

Interestingly now that it's "GUID" not "Master Boot Record" it allows me to add the disc to a RAID list (not that I need to now). It's like you said purana, the whole disc not partitions, I got confused when it says "Drag discs or volumes to the list box".