I was looking for an adapter that would act as a bridge from USB to Pro Duo, when I couldn't find one, and decided to try to make one on my own. Now that firmware 2.8x emulation is out, with something like this you might be able to have an 80gb hard drive that your PSP can access (or a pen drive).
Here's an image of what I'm talking about.
As you can see, it's really quite simple. It would be nice if a company would create something like this but smaller and all in one piece. For now, you can create one of these yourself. I've laid out a few pointers below:
Creating the fake Pro Duo cards
You know those Pro Duo to Duo adapters that come with new memory cards? You'll need two of those, as they contain this:
You only need to take apart one of them completely (take off metal cover, then find and take off the plastic weld), attach a thin ribbon cable or wires. As the Duo Adapter case will not fit in your PSP, you need to create a mold: mold it with plastic (I suppose) The mold itself could just be a clay mold of an actual Pro Duo card. You need to mold it so that the fake pro duo card is strong enough for you to be inserting and taking out of your PSP by pressing on the top (is there any other way?) Otherwise the fake card might get stuck in your PSP, or might break within your PSP.
As for the other Pro Duo to Duo adapter; you only need to take off the metal cover off it, which will expose the little pins. If you really wished, you could take the pin board out and create a mold, but it isn't necessary, as the Duo adapter should fit into your Duo/Pro Duo to USB adapter (most take both sizes).
You're probably wondering why we have to use a Pro Duo to USB adapter, and not create your own. The reason is because Pro Duo cards are parallel, while USB is serial. Apparently the require some sort of converter... good luck finding one or making it on your own.
Note about the cable between the two fake Pro Duo cards:
1. The best thing to use for this cable is are thin ribbon cables used in laptops. I'm sure you can find one that matches the pins. If you do not have access to these, you should be able to use regular wires/ribbon cables (though you may need to strip off some of the plastic from the wires for it to remain as thin as the fake card).
2. I can see people making this mistake right now, so make sure you line up your wires properly.
I'm starting to think that power may be a problem. Normally, Pro Duo to USB adapters get their power via USB. However, in our case, the USB is the pen drive or hard drive, which the adapter can't get power from (if needed?) It may be able to get power from the PSP, but... off battery power that wouldn't be such a great idea. You may need to wire a small 5v battery into the usb cable.
I don't think power will be much of an issue, so you can probably ignore all that I've said above... except that it's something to consider if it doesn't work right.
Honestly, I haven't done this yet. But as you can see, I've planned it all out. I just haven't had the time to do it, or to create the mold (probably the hardest part). So, I am permitting you all to use this idea, distribute it, get the idea out to companies. Like I've said before, I really wouldn't mind a company to create such a device in a smaller, simpler form.
I'm not much of a hardware/electronics person (though I'd like to be). Please, tell me if there's a flaw in my idea, or a better approach to do something!