seeking 6TB Ext drive for PRO

I am looking for a less expensive way to expand my TiVo External Storage than going to WeaKnees $900 upgrade.
How can i configure an external drive so it will work with TiVo Roamio Pro?
I was under the impression that if I got WD30EURS or WD30EURX and place sit in the shell of my WD ext 1TB expander drive that it would work.
WD says no.