Dead Disk.. but not really

Hi, I have read all the post with interest as I am in similar position and getting nowhere with Tec support. I purchased the READYNAS Pro 6 and started off with 3 drives, Seagate ST2000DL003 was the choice. I setup the Nas without incident and then sent off for another 3 drives. During the delivery period 2 of the drives started playing up and reported after a while as "Dead". Seagate will not take back drives without you run their test which I did, and the drives both "Passed" but refused to be acknowledged in the Ready Nas. I opted to backup all the transferred data and do a full reset. After 1 hour both drives failed again. I did this twice more with the same reults and then gave up. I then loaded the Nas with 4 good drives and did a full reset and everything has been running fine for a week. I have sent the faulty drives back for replacement with full explanation of the problem but have yet to see if they will oblige. I contacted Tec support to ask for advice and frankly they just don't get it, and trot out the standard blurb, which is unhelpful. As I see it, the software on the NAS which checks the drives must use different test criterior to the Seagate or Windows programs. I asked Tec support for a link to the software the NAS uses with no joy. I also asked for other methods to test prior to adding drives to bays 5 and 6 and risking further problems. I have read other reports critising this particular drive model (this is since all my problems began, where were they before I shelled out?) and wonder if the Seagate ST2000DL003 should be on the HCL.
My main questions.
1: with Xraid 2 my understanding was I can start off with one drive and incrementally add others, up to the max of six. Auto volume expansion will just do it's job. How do you do this in reverse if you have a problem with a faulty drive? ie take the drive out and tell the volume to adjust itself to the new situation?
2 If that is not possible how do you test your new drives BEFORE putting them into the Raid with the same software that the Raid uses?
3: Should I take out a running drive and replace it with a different drive as a test maybe?

Sorry for such a long question but can anyone offer some realistic help and not the standard replies in the manual please?
Regards Martin