Could I run video games off of a USB 3.0 External Hard Drive?

I've done the original counter strike off a USB stick and that worked.

Depends on the game really. Some will work, some won't. More demanding games will typically not run as well from an external harddrive or Flash drive though.

Edit: Though, I suppose I should clarify a bit. If you're just gonna keep it connected to the same computer it should work with any game. What I mentioned previously is mostly only referring to if you plan on taking a harddrive with you for different computers. I just kind of assumed that while typing my original response.

Yes.

yes its possible doing it with a Seagate

Low end games like SSF2 (look it up) run great on USB 2.0, but I am not so sure about stronger games. (P.S. I own a U.S.B flash drive ((16 GB)) So I should know.)

USB 3.0 > Sata 2, at least for transfer rate, but I'm not sure for seek. In theory, it should be better than anything that isn't internal/external sata III / usb3. This is just a theory though, I'm geek at best, not a technician :D

Of course you can. You could on USB 2.0 too.

Storage speed only really dictates loading times, as data is moved from storage to RAM. Unless data is being streamed of course, but I can't think of any PC games that really do that besides Rage.

I'm doing it with a Buffalo brand USB 3.0, so you can.

For the most part, most games can run fine from an external drive/USB stick. Games with large files that are needed to load such as BF3 might experience loading stutter. I say might, because it depends on the internal specs of the drive, fragmentation and the workload of the rest of the PC's interfaces as in do you have a lot of other USB devices using resources?

Overall, give it a shot. Small games like Terraria can easily be run off of a usb stick via USB 2.0 or sometimes even 1.0 though I havnt seen any 1.0 interfaces in the wild for sometime.

MMO or MP games have a much higher chance of seeing this load stutter since you have to stream more data from the drive then you would normally have to. This is from my experience though so its best to test it out yourself.

It will work just fine, though SATA is faster.

Games that load most data in one go will work, but games that constantly read off the disk will not. I had some issues running JC2 off an external, but one of my faster flash drives was no issue.

I do it often with and without Steam.

Yes, however because of read/write rates I would recommend a large capacity flash drive in lieu of a mechanical external hard drive.

Something like this: http://www.newegg.com/Product/Product.aspx?Item=N82E16820233326
For example, which has a speed of 220 MB/s read and 130 MB/s write, which is 2 to 3 times the speed you're likely to see with a USB 3.0 external HDD.

You can play most games from a USB 2.0 drive. The majority of mine are on one 1TB portable drive. Toshiba Canvio, from four years ago.