Big portable USB-powered hard drive advice? (Or use a cloud????)

I want to add this to my daily use machine, which just has an SSD inside. I'm tired of skipping some music due to no space, and laziness to get/plug/connect an external drive.

Plus, one of my backups won't spin up, so while I may try to rescue that it is time for more redundancy.

Does anyone know of a large capacity, small size drive powered entirely by USB? I want it small to velcro onto the foot of my iMac, more or less permanently connected. My current backups are 2TB and getting full, so I'd like to find a 3TB or even 4TB model.

Hmmm, if it had FireWire that would be cool but not a must.

I don't *think* there's enough cloud storage-not for several terabytes of stuff, eh?