I contemplated such things and tried them.There is device - multimedia player/viewer from company - "Vosonic"VP8870,which has max capacity of 640G,and it takes 2.5inch drives.You can buy it empty,and you can change drives.
You can also use pmp with OTG function - you can connect external drive to pmp.I have pmp Teclast TL-C700,which works well - for videos;another works better for music:Ramos T12HD.
You have to power hdd!,pmp battery is to small for this,but you can prove the concept.Practical use on the go?(OTG stands for "On The Go") - in car,on your boat,in train if you travel on shinkansen,in airplane.
Not much joy,when you are walking.
You can use also - Pandora computer,you may find info in another thread.
In Cowon - ? only Q5 may work.D3 has OTG,but it works only - copying hdd to the player.
The easy thing to do - use ultraportable notebook or netbook.