I just saw this thread and read all of the responses. I think I can help clarify.
The main difference between a "standard" and a "portable" external hard drive, aside from the size, is the typical lack of an AC adapter (or at least a lack of its necessity) on a portable hard drive. The first external I bought was for a notebook, and I wanted something small that I could store movies on for my commute to and from work on the train.
These are great when you're on the move. But while the major benefit is their portability and only needing one (USB) cord for it to function, there is also a major downside.
The typical "portable" external hard drive owner bought the drive to use with his or her notebook/laptop/netbook, etc.(for use with their "portable computer"). Portable drives are typically considerably more expensive than standard external drives of the same storage capacity. Both drives need both the power to operate from an outside source and a data cable connecting them to a computer through which to send and receive the information they store. So portable hard drives are getting both jobs done through one cable when operated without a dock.
A major issue here is that the typical notebook has a very limited number of USB ports. I have a year old mid-range HP notebook and it only has two ports. So most people add a USB Hub to connect additional devices. Unfortunately, with an unpowered USB Hub (one without a separate AC adapter) you are just distributing the little bit of power that the USB port supplies over however many ports the HUB has. So if you have your FreeAgent Go hooked in to the USB Hub with it's one little USB cable, you probably won't have enough power to run a second hard drive on the same hub. Ipods, phonesdigital cameras, etc. all compete for the little power and data transfer bandwidth that is available.
Even with a powered Hub, too many poer intensive peripherals really bog the whole system down quick.
Once I figured this out, I began buying standard external hard drives to use at home, and I had my one portable for the road. I didn't think I had enough power to run the Freeagent with my two standard 500 GB seagate's connected with everything else. I also really didn't understand why that extra USB connection came with the portable drive.
Then it hit me one day. I couldn't find the regular single USB cable for the portable drive, and I wanted to transfer some TV shows from one of the standard drives to the portable. I had used the dock before with only one cable, so I tried it - and nothing happened. When I looked at the drive, the lights were on but my PC didn't recognize the hard drive. So I switched to the other cable and it worked. Then it hit me like a ton of bricks.
The only way the Dock works with only one USB "plug" inserted is if you use the plug closest to the dock. It has a fatter cable going to the dock. From the plug that "works" to the other plug is only a few inches, and the cable is thinner. The reason the cable is thicker from plug that works alone is that both the power and the data cables are running between it and the dock. The far cable only has power wire running through it.
The far plug, the one that does NOT work alone, is ONLY a power source for the dock. The second cable in transfers data AND receives power.
Now, I leave the dock plugged in & set up at home at all times, and keep the little cable in my computer bag. Because I'm low on available power via my (powered) USB Hubs, I tried successfully to attach the freeagent go Dock's near USB "plug"(The one that works alone) to my USB Hub, and I plug the far (power only) USB plug into one of those AC - USB adapters. The simplest and cheapest of these looks like an old motorolla cell phone charger - the fat part that plugs into the wall. Except, instead of a wire coming from the bottom to attach to your cell phone, there is just a single(on the cheapest ones) USB port. These are designed to power USB devices whose primary power supply for their batteries are a USB connection(like MP3 players).
Just to be clear, I never heard, read, or was otherwise informed by Seagate that this is an acceptable configuration. I have had mine running like this for a while now, and it was IMPOSSIBLE to run with both other hard drives plus my other peripherals running at the same time before I tried this. In my eyes, it basically turned my portable drive into a "standard" drive at home, while still keeping it's awesome portability.
I hope this helps. If my hard drive blows up, I promise I'll let you know right away. But those AC-USB adapters should provide the same power level as the USB ports on your computer - before it is divided by a HUB and multiple devices attached. But, if yours blows up, don't get mad at the site. You shouldn't have placed so much confidence in the logic of a semi-anonymous forum poster whose most recent major accomplishment was realizing that one wire is for power.
Good Luck.
