Did you have first revision (R210) or second revision (R210 II)?. R210 supports two hard drives. You can refer below link for more details [URL]  Below link have an old post on same topic [URL] 
Can you put 2 hdds in a dell poweredge r210 1u server becuase i need to make a nas with 2 hdds in it.
2 hdds on Poweredge r210 1u