ReadNAS 400v2 repair manual

You can add drives to any ReadyNAS in any order.  Across, up-down, or skipping drives for cooling enhancement.  Note that the rack mount units usually come with "air dams" for use in slots without drives, and a used unit probably won''t include them

The 4200V2 is a modified SuperMicro system with a slightly modified X8SI6 motherboard and a BPN-SAS-826TQ backplane.  Documentation from SuperMicro is applicable except for the IPMI interface, which has been removed.  CAUTION:  Don't upgrade the BIOS with just anything you download from SuperMicro, as it may not work with the ReadyNAS OS.  The OS does verify it's operating on legit hardware.

There is a huge difference between a 4200 and 4200V2.  Besides a slower processor, the 4200(V1) only supports 2TB drives in 8 of the 12 bays.  I suggest you avoid it unless it's seriously cheap (which isn't usually the case because of shipping).  Same goes for the 3200, which has the same motherboard as the 4200(V1) with an even slower CPU and less RAM.  All of them use power supplies, fans, caddies, etc. for SuperMicro 2U chassis, which are readily available on eBay.  The SuperMicro "dummy drives" for unused bays can also sometimes be found at a reasonable price.  I've even used them in an RN4220, which is not a SuperMicro derivative (It's Chenbro, but much more customized). 

The 4200(V1) and 3200 are also OEM SuperMicro units, but I forget which motherboard they use.

The 4200V2 is, IMHO, an excellent NAS when converted to run OS6.  I run a converted RD5200 (same motherboard, some other differences) as my primary and 4200V2 as backup.  They are, of course, noisy beasts like any other rack mount, so you need an appropriate place for them.