DAC's and external portable hard drives

The answer to your query is yes, this is a common feature offered by many DACs

My Oppo 105 had a 2TB Seagate HD attached by usb to it.  I have a Marantz NA 7004, which actually is both a DAC and a Media Player, which does the same thing but very poorly since it is buggy as all get up.  
  I have 2 other DACs—Mytek Manhatten and Bryston DAC-3–that have USB inputs that theoretically could play usb HD, but the problem is controlling the content.  That’s where Media Players come in handy.  With the Oppo, I eventually bought a 19 inch monitor to attach to it which I only use to help navigate the various HDs containing music that attach.
  I think what the OP probably wants is a combination DAC/Media Player.  Lots of choices here.  Cambridge Audio, Bluesound, Aurender, Auralic, Naim..... I would scan the offerings of a company like Audio Advisor or Music Direct.
  Of note in the Home Theater arena, most AV receivers or Controllers do this routinely, and already have monitors to help navigate (your TV set):

Without knowing your budget, it’s hard to get more specific with recommendations., since  the range can be from a few hundred to the tens of thousands of dollars .  From what you describe in your post above, you’ve been living quite happily without a DAC, and there won’t be a lot of files played here.  I would therefore opt for an economical option that will sound good, and if you get the file playing bug you can upgrade going forward.
  I would investigate the Audioquest Dragon Fly.  This is a DAC that looks like a usb drive.  It attaches to a usb slot on the Mac.  You then need a cable that attaches from the Dragon Fly to the inputs on your pre amp.  Total cost should be between $150 to $350, depending which flavor you buy.  You can then play the “stray digital tracks” from the PC into your system (or headphones).
  I would compare the Dragon Fly with the Bluesound Node2.  As mentioned previously the Node2 comes with an included DAC.  It will recognize your Mac.  It will also play Internet Radio—if you haven’t investigated the offerings here, then you are in for a pleasant surprise—and also every streaming service imaginable if you decide to add one.  Cost around $500.  It also has a very useable app so that you can navigate your files from a portable device