Duo v2 Backup Job: Input/Output Error, Unknown Reason.

Hi,
I have a client who has a ReadyNAS Duo v2 on which I have set up two External Western Digital MyBook 3TB drives (WDBACW0030HBK) to act as weekly rotating back up drives.

Initially the client only used 1 of these drives to back up all the contents of their NAS to by way of attaching the external hard drive to the front USB port on the NAS and scheduling a backup job. This worked fine, the client then wanted to introduce a second hard drive, so they could keep one onsite during the week and one offsite and rotate them.

I tested this by just plugging in another external hard drive that I already had (a Seagate 2TB drive, can't remember the model) and running the backup job. It worked fine. So I ordered the client another Western Digital MyBook 3TB drive which was identical the one they were already using. Now when the back up job runs, it gets half way through and fails and sends an error log, which is as follows:-

Error encountered copying data from source path  ==> /USB_HDD_6/ due to unknown reason.  Please see log.
cp: writing '/USB_HDD_6/./Company_Data/Quotations/JOHANNSEN - QUOTE 2505.pdf': Input/output error
cp: writing '/USB_HDD_6/./Company_Data/Quotations/SMITH - QUOTE2502 SOUTHWATER DR.pdf': Input/output error
cp: writing '/USB_HDD_6/./Company_Data/Quotations/BACKEN - QUOTE 2533.pdf': Input/output error
cp: writing '/USB_HDD_6/./Company_Data/Quotations/RUSS - QUOTE 2569.pdf': Input/output error
cp: writing '/USB_HDD_6/./Company_Data/Quotations/SIMS - EDMONDS QUOTE 2580.pdf': Input/output error

I have clipped most of the message as there are 100's of lines of the same thing.

I have tried formatting the drive and updating the firmware on the NAS which doesn't seem to have made any difference.

Any ideas?

Thanks,
Ben