I'm using the B model pi which has two usb ports. I originally had 1 1TB external usb hard drive connected to it and then tried to add another 1TB drive and it seemed like that one would not power completely (it sounded like it was skipping).
When I swapped the hard drives (only one connected now), it worked fine. I'm using the charger from my phone (5v === 2A). What power source would I need to make them both preform together?
Have a look here:
Each of the two USB ports on the Pi has a polyfuse rated at 140 mA, so any connected USB devices should draw less than this amount of current. In addition the polyfuse will cause a significant voltage drop, so that USB devices get less voltage than is available on the RPI itself, sometimes up to half a volt less (maybe more if the fuse has recently been hot).
In other words, the pi is not designed to power anything significant from the USB ports -- technically a USB port should provide 500mA, but the pi's are limited to 140.
So, get a powered usb hub. If you get the right one, you can power the pi itself too; while the USB standard dictates 500 mA per port (no less and no more), some hubs do not regulate this and will output whatever up to their total amperage rating. I had a look through here last week for one people reported as useful toward this purpose, and can confirm that the "Belkin F5U237" 3.8A 7 port hub does not seem to have any problems! It was easy enough to find locally but cost $30; there are smaller ones on the list that apparently work too. I would assume if Belkin don't regulate one model, they probably don't regulate any of them.
It's nice having the upward facing ports to smack a keyboard into occasionally too.
