ReadyNas marking brand new drives as dead

I have a NAS unit readyNAS nv+ with 4 WD 2Tb green drives.

One month ago drive 1 showed as failed, the raid went to unprotected state, then one week later drive 2 showed also as failed, both showed dead state.

I bought 2 x WD red 2Tb NASWare3.0 drives. Putted one of them back, resync, then the second one and resync, everything showed working and Frontview gave me green lights, I could read all the data, so I didn't do any external backup or anything, I just assumed drive 1 and 2 where dead but as I had 4 working drives and a unit giving me green lights I assumed everything would be alright. 

Around 1 week later Drive 1 failed, and next days Drive 2 failed also, both showed dead (brand new drives mind you). 

I lost all the data this time, could not rebuilt nor access any data :( I should have backed everything but did not, this is not the current subject of my post.

I put all 6 drives(4 green and 2 red) on a computer and formatted and reinitialized all of them. I replaced all four WD green drives back in the unit (I kept the 2 reds aside). I changed the order of all the drives, so that the drives originaly market as dead went into space 3 and 4.

After the factory reset all drives showed green and ok in frontview, I did copy some files and everything seemed to work properly, then 24 hrs Drive 1 showed as failed... the current drive 1 was before the drive 3.

It would seem as if any drive I put into slot 1 or slot 2 will be recognize as dead (after a few hours/days of usage). 

What should I do ? What's causing this problem ? currently if I believe what the ReadyNas is telling me I have 3 green faulty drives + 2 brand new red ones also faulty ? I don't think the problem is with the drives, it would seem as if the unit is not behaving correctly.

Please advise.