LS-QVL - Does not recognize / rediscover additional drives

[gazza040502]

I have 2x4TB drives currently setup as Array 1 using RAID 1 (disks 1 and 2).  I have 2 spare drives (2TB each) out of an old D-Link NAS that I inserted as disks 3 and 4 but the Linkstation fails to recognize them - I wanted to configure them in an additional RAID 1 array (Array 2).  The green LEDs come on on the front of the unit, but in the web admin interface those last 2 disks always show as "removed".  I have tried selecting and hitting the "rediscover disk" option several times but nothing changes, all other options are greyed out for those disks. 

I even tried inserting an old 320G disk (readable in a USB caddie when attached to Windows) but again it is not discovered. 

Is it actually possible to have 2 separate Arrays of RAID 1 in this model of Linkstation? 

Any ideas why these additional drives do no show?

Model says LS-QVL/R5 series in NAS NAvigator 2
Firmware version 1.71 (just updated). 

Thanks,
Gary.