readynas6000: drives 1,2 and 3 of 6 not detected

It sounds likely you bought a bad NAS, especially if it was sold as-is, no return. But maybe it's just a display issue.  Download RAIDar from ReadyNAS-Downloads and verify it reports the same thing.

There is the possibility you have some bad drives, even if they are new.  It seems unlikely you would just happen to put the bad ones in bays 1-3, but one bad among those three could make the others appear non-functional.  It is also possible, but unlikely, that the power supply is failing in such a way it can only power up some of the drives.  I suggest you swap the drives between the working and non-working bays and see if that changes anything.  Moving a single drive that you know works from bay to bay can also help determine if the chassis is good.  If all bays do seem to work with some combination of drives, then testing the drives themselves with vendor tools is the next step.

If it is the chassis, it could be the main board or the SATA backplane, but it's impossible to know without swapping each out, and the boards are not available from Netgear, anyway.  If the NAS is non-returnable, report back what you find with the above tests, but there's probably not a lot of hope.  Of course, it could still be used with 3 larger drives, at least for a while.