^ Don't buy cheap plug packs from China (you get what you pay for). Plug packs that are only a few dollars will be 'noisy' (have poor regulation due to design shortcuts) and possibly damage your router.
I'm guessing 'AC Input: 110V-240V (50-60Hz)' and 'DC Output: 5V @ 2A/2000mA' are printed on your router's power supply at present (based on the unit's specs on the Asus website).
V = volts (unit of electrical pressure); A = amps (unit of current/electron flow).
In terms of water flowing through a pipe:
- volts can be compared to the water pressure
- amps/current would be the rate of flow (depends on supply demand and resistances [which could include the diameter of the pipe itself)
Basically, your router requires a 5V plug pack (another name for a power supply) - it can't be any other voltage. A lower of higher voltage would affect the 'electrical pressure' (again in terms of water flow, putting '12V' of water through a pipe (your router) designed for '5V' would cause it to 'burst').
Current is different. It's varies based on demand. Your router's specs state that it may consume up to 2A, so you will need a 5V plug pack capable of providing at least 2A.
A 5V 3A (or even 10A) plug pack will work, however I actually doubt you'll get extra current through the USB ports as they'd most likely be internally fused (if the router has been designed to standards) and not allow more than 500mA through each port (router electronics would use about 1A). That's probably why the hard drive was 'malfunctioning' also. But I could be wrong about this, and since a (5V) 3A or 4A plug pack won't be much more expensive, perhaps you could just go for that. (There are tests you could do to see if the router's plug pack connector was wired directly to the USB port power lines, however that would involve having to pull the router apart and using a multimeter to test connections - not a simple process).
If you're in the US, I'd personally purchase a plug pack from Radio Shack, where I'm confident I would get a quality item.
Finally, your current power supply should have this logo on it, indicating that it has a connector with a centre-positive tip: (there's a 0.001% chance it won't).
If you get a supply with a reversible plug, (as stated in another post above) make sure it's on the right way (ie. + aligns with the word 'tip').