I don't have a 645, yet and I looked at my 657 and it has a Media Server settings under Setup. Does the 645 have this option?
Yes, under advanced setting there is DLNA Settings and ITUNES SERVER.
I tested the DLNA Settings and it's working

1. Checkmark the
Share media libraries with devices2. Name your media library: e.g. DIR645_DMS
3. After save settings I could browse the external USB harddrive and search for the folder with music
4. After saving the settings I found
DIR645_DMS under network >>> multimedia
5. a double klick on DIR645_DMS opens the Media Player where I can play the mp3's (under other media libraries) from that external harddrive
Good to know despite I'm using the external usb harddrive with shareport software.