PeterT, according to the information you provided on your post the problem started once you replace the 2nd hard drive (1TB). We had seen this behavior before and it was caused by a lack of power.
It could be that these 2 new 1.5 TB HDD are demanding more power than the previous 1TB HDD.
You may want to check with the HDD manufacturer, which is the minimum power requirement.
You can also send me a print screen of how the system is seing the HDD on the Ctrl+i menu.