You need one which supports an option for use as a removable disk (sometimes called MSC) in its PC connection settings. This allows simple file drag and drop between player and computer.
Tends to preclude players which depend on proprietary software or Windows Media Player such as the iPod, (I believe) the Zune, some Philips models and earlier Sonys.
The snag in your plan may be speed -- even with USB2, transfers may not be up to the pace of a dedicated portable hard drive. The drive in a player is designed to be as small as poss and speed of transfer isn't an issue when playing music.
i was going to buy a portable hard drive so i can move all my music and videos from work to home, to friends houses easily. But then i thought if i buy a big mp3 would it be able to do the same thing?
I've gone off ipods after my computer crashed and i couldn't even get the music off my ipod and put it on my new computer. but can you do that sort of thing with a mp3 player? or anything that will also play music

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