You can expand the storage capacity of an MMS by connecting a FAT32 or exFAT (as of MMS OS 6.3) formatted external hard drive via USB.
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See this link for an explanation on how to access the server's media shared folder: http://www.autonomic-controls.com/kb/index.php?View=entry&EntryID=129MMS - Access Music Share
See this link for a list of supported devices: /wiki/spaces/ASKB/pages/1580302337
Note: The ADD/MOUNT options may not be available with portable USB drive types (e.g. thumbdrives)