Hello, mathewlister. Hopefully I can solve this for you, and the other guys. In the owner's manual for the MD3400 / MD3420, it states on page 43 that you should be using an MD1200 / MD1220.  It's in the 'Technical Specifications' section. "Two SAS IN ports to connect hosts One SAS OUT port for expansion to additional PowerVault MD1200 or MD1220 expansion enclosureNOTE"  Found here: downloads.dell.com/.../powervault-md3400_Owner%27s%20Manual_en-us.pdf I hope this helps. Let me know if you have any questions. 
Hi All we are looking at buying a MD3400 but need around 100TB of storage so want to bolt an additional shelf to the unit, can someone confirm the additional shelf which should be specified with this? We have had conflicting information stating it should be the MD1400 which is 12Gbps SAS although some have stated it should be the MD1200 which is 6Gbps. logic would say it should be the MD1400 to match the 12Gbps back plane of the MD3400, the conflicting views have all come from within Dell?? if someone can help confirm the correct solution that would be a lot of help Thanks Mathew 
Dell MD3400 + Additional Shelf