I assume its an HP Stream if its 32gb because I have one (which is what I am typing with right now) and I'm just saying, even if you are gonna get a hard disk it's cpu isn't that powerful, can barely run Firefox on performance mode.
Anyways, I would get a micro SD card . If you are even getting a stream, the micro SD is gonna be the most portable, just guessing because of the tiny storage size which literally only has 2 gigabytes of space free after system files are involved, but the laptop is very very portable. I will start looking and let you know if I see a good micro SD, but from what I've heard its possible to get small hard disks with a storage box that sits on the side. That might be your best bet if you want a reliable storage unit.