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Connect the external drive and then navigate to the server's config page under the contents tab. There are 3 options for expanding storage:
ADD - This will format the hard drive (deleting all content on the hard drive) and expand your MMS's capacity. The sync tool needs to be configured with a local computer, in order to distribute content to the external drive.
MOUNT - Will use the existing drive as is, without formatting. This requires the drive to be pre-formatted with a support file system. Existing music on the drive will be available for playback. In order to add content to the external drive, please access the server's media shared folder and add your content to the folder labeled "_Mounted"
IMPORT - This will import all the content from the external hard drive to the MMS's internal drive. This option will not expand your MMS's storage size
See this link for an explanation on how to access the server's media shared folder: MMS - 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)