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Anything that takes SD cards should make storage a non-issue and make file transfer dead simple. Even microSD cards can hold over well over 5TB

Dedicated players are getting harder to find though. I had a Cowon mp3 player for a few years which was pretty nice. Maybe see if they've got something you like? http://www.cowonglobal.com

SD cards of that size may not be compatible with many devices though.

I just rotate music on a microSD for mobile. However, you can optionally self-host a music server and connect with a client on your device to stream. It's also possible to SSH from outside home network, if you really want to.

iPod video 5th gen w/ Rockbox custom firmware. Then do the bluetooth mod and you're in music heaven. Probably fairly easy to write a plugin for the functionality you mentioned.