I wanted to ask this question before, good thing you brought it up.
I'm hearing that to access the files through the chromebook file manager you have to wait till it's finished reading the whole drive and could take a while, about 20-40 minutes.
So if you do get a external hard drive, getting something with smaller storage will be better. 100-200 GB's shouldn't be too bad. I have a 1 TB Western Digital HD and from what im telling you, i will not be using my 1 TB Hard Drive on the chromebook.