"Portable Drive" often is taken to mean one that is small and needs NO power supply module, since it gets all its power from the USB2 port it is plugged into. These are often used with laptops. They work that way, but ONLY because the HDD unit inside is specifically designed to run with the limited amount of power available from a USB2 port - max 500 mA. Almost always that means the HDD is running slower - typically 5400 rpm. So if you look for a complete external hard drive in the small 2½" form factor, you will find it really hard to get one at 7200 rpm.
You can get an external enclosure and mount a faster small HDD inside - say, something like a WD Black 7200 rpm unit typically used inside a laptop - but it will need more power than can be supplied by a USB2 port. IF you are ONLY going to use it with a USB3 port at all times, it can work - those newer ports can provide more power. But unless you're sure of that, you would need another plan.
The simplest plan is to buy an external HDD enclosure for the HDD unit you want, where the enclosure comes with its own power supply. Usually that is in the form as a "wall wart" unit that connects to the enclosure, although a few are inside the enclosure. With these enclosures, no power is drawn from the USB port. Now, if you go that route, you have one more degree of freedom. You could buy a larger enclosure (instead of the small ones for use with laptops) that accepts a "standard" 3½" HDD (internal) unit, probably SATA. Such enclosures are larger, but still portable - they can be carried easily. This has two huge advantages: (a) the 3½" HDD units are much cheaper per GB than the smaller 2½" laptop units; and, (b) you can get a 3½" unit with much larger storage capacity than the largest 2½" unit.
You should also be aware of another factor you have not mentioned. Above I have assumed that your connection between the computer and the external drive will be by USB. (The alternatives, eSATA or Firewire, don't provide power to the external drive through their ports, so with them you always will need a power supply module for the enclosure.) Now, of those three interfaces, USB2 is the slowest. (USB3 is as fast as, or faster than, eSATA or Firewire 400.) In fact, the data transfer speed of a USB2 port is substantially slower that the speed of the HDD inside the external drive unit. So, if you will be using USB2 as your connection system, the difference between 5400 rpm and 7200 rpm in the HDD itself will be wiped out by the slower interface speed, and you'll never see the difference. If you want faster data transfer, you need more than a 7200 rpm drive and an enclosure and power supply system to match that. You also need to use a faster connection system - USB3, eSATA, or Firewire.
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