You could add another file (create an MDF file and/or an LDF file) on the other disk(s) and expand the DB to use it, but this becomes cumbersome if you do not have a specific plan for spreading your DB files across disks. This is usually done for performance reasons.
You could back the DB up and restore it to the new disk. You could detach the DB from SQL Server and move one or more files over to the new disk then re-attach them (this one is probably the best, easiest, and quickest solution).
Look up sp_Detach_DB and sp_Attach_DB in Transact SQL help or BOL.
Chris