Search the forums for "2nd HDD". This has been discussed before.
And for the record, SME only installs on a single HDD (not counting RAID). However, after installation you can customise SME to support as many hdds as your system can handle (within Linux working parameters, of course), including configuring your second HDD to hold the ibays, etc.
A word of caution, only connect the additional HDDs AFTER you finish loading SME onto the server (otherwise, SME's installer will wipe them all out !).
Kelvin