[Motorola Droid Bionic] Vadr64:
Disclaimer - This is my first post on this site, so I'm going to try to be strait forward and concise, I'm sorry if any info I give lacks a certain panache.

First and foremost, if you want to use a hard drive on your phone through an HD dock, you must format your hard drive to Fat32. This is a very basic, but unavoidable step. That is if you don't want to make the entire process way more complicated. I used Easeus Partition Manager to format my entire 250GB hard drive (impressive I know). You can use other partition manager software, but Easeus is the one I'm used to, and I found a free download of it, which works perfectly on my old XP PC.

The Second step is to make sure you have enough power to make your hard drive usable, and thus be recognized by your phone. I used a USB hub with a Y-Cable and my phone charger to supply the extra power. Note: Do not put the additional USB power port on the HD dock, you will not get enough power. You have to use an additional power source for this method to work. I suggest using at least a 5V 1A charger, but my phone charger is 5.1V 850mA, and works perfectly.

Step Three is to connect the hard drive. This may seem like an unnecessary step, but there is some important things to know. Do not plug multiple storage devices into the hub. There seems to be some priority conflict when this happens. Thus, if you have a Thumb Drive and a Hard Drive, connect the Thumb drive to the HD dock directly, and the Hard Drive to the hub. I haven't tested if there is a similar conflict with two hard drives being plugged in to the hub simultaneously, nor two thumb drives. There also seems to be an issue where if you only have the hard drive plugged in, but no other storage device in any port, the hard drive still won't be detected. So, you may need to plug a thumb drive to your HD dock, and then a hard drive to the hub. This results in your phone calling the Hard Drive the same device name as the Thumb Drive, but the two are shown separately in the file manager.

The hub I used was a Micro Innovations 4-Port USB hub [Model # USB210P/USB210P-F], with a Mini USB port for the main data connection. As such, my Y-Cable was a Mini USB to Full USB Y-Cable. My USB enclosure for my hard drive was made by Manhattan, which has a single USB power/data connector. My Thumb Drive was a 16GB SanDisk Cruzer Micro. My additional charger was my stock Droid Bionic charger. I also used USB extension cords as needed.

In regards to my use of a hub and a Y-Cable there are a couple simple alternatives. One is to use a Y-Cable with a full sized female USB connection in stead of a Mini USB connector. This would allow you to forgo the USB hub. The other alternative is to, depending on the hub you have, use the provided wall charger to directly power the hub, thus not needing the phone charger, nor the Y-Cable.

This method of allowing your Hard Drive to work on your HD dock should work on other devices, as well as (with minimal alterations) silversonic1's guide "How to use USB Host without an HD Dock." http://xdaforums.com/showthread.php?t=1495720

Please let me know if this helped you, and feel free to post any additional info on your success and methods used.

Edit 05/28/12: My brother bought a power converter that gives 5V 2.1A, or an optional 5V 1A on two ports. It's an RCA AC to USB converter, model # PCHUSB2R. This adapter was able to supply enough power to my hard drive for it to be read without my Thumb Drive being plugged in as well. It appears that the biggest problem the HD Dock has is that it does not give enough power to charge the phone and power a hard drive through USB. The normal power supply provides 12V 1.5A to the Dock, but I don't know how that power is distributed throughout the Dock. This apparently causes problems, but the additional Amps from the new adapter resolves a lot of them.

Although, recently I've had the problem of the Hard Drive failing on the phone's OS, but not on the Webtop OS, so the hard drive is still usable where it needs to be. You just have to go into the Webtop's file manager to unmount it once your done, but sometimes it won't say it's unmounted, so be careful. Also, the hub solution still doesn't allow you to plug in extra storage devices. It seems you're limited to one storage device per USB port on the dock itself. So, a max of 3, while, theoretically, you should be able to plug in up to 7.