http://www.microsoft.com/technet/prodtechnol/exchange/2003/insider/transdb.mspxthere is a section in there
"The .stm or streaming media file is used in conjunction with the .edb file to comprise the Exchange database. Both files together make up the database, and as such, they should always be treated as a single entity. "
so as far as i know they have to be on the same drive.