A Virtual Hard Drive is a file that simulates a physical hard drive. It allows you to install software from within an emulator, this also includes games. These files are usually .vhd or .sdc. Installing a program with a virtual hard drive lets it run faster and makes the whole process seem more natural, especially if the program is an older one.
How Do I Create a Virtual Hard Drive for My Emulator?
There are many programs out there that will let you create virtual hard drives. An example is games like GBA and NDS, which use .gba/.nds files to run the emulator. These files have been translated into English from their original language so it's not much trouble to find them on google. The one seen above is an example of a program called SD Formatter which lets you create .sdc files for your virtual hard drive. It also comes in handy for formatting your memory cards if needed, but I never do that personally, just use my real ones or emulated ones in the GBA/N cases. The other option would be to use the hard drive .vhd file and manually extract what you need from it.
An example of a virtual hard drive for GBA/NDS is seen above. It's called Free Mcboot and it's used to install all your homebrew games and applications. All you have to do is put this file into the root directory of the Sd formatter or just extract what you need from it using 7z if the program isn't recognized by your computer [which shouldn't happen]. After that, just download the sd formatter [and extract] and run it [no installation needed]. You should be able to see something like this:
After clicking on 'Format Size Adjustment', make sure the Gamecube Mode is set to none and the file system is set to FAT. It takes a few seconds but it doesn't really matter, so don't worry about watching it. The next page should pop up and by default, there's no 'Boot File' so just click on 'Add New', which will open up this menu:
Now you just have to type in the name of your virtual hard drive and the source file. In this case, we're using Free Mcboot, but since I'm pretty certain everyone knows what that is by now I won't post a screenshot of it. You can also use Freeloader or Swiss if you want those programs instead; they work exactly like Free mc boot as far as installation is concerned. Now once you've typed in the name, everything else should be good to go. It'll say 'Boot File Found!' so just click on 'Apply' and you're done. The next time you want to use your installed homebrew apps or games, enter into emulation station and it should boot up normally if you entered in the correct name.
This virtual hard drive can be used for any GBA/NDS game/emulator even though I only tested Free Mcboot with these steps but this process is pretty much universal for all emulators that require a virtual hard drive. You can also make multiple ones if you use different emulators like Dolphin, Nestopia, Fceumm, etc.
Benefits of Virtual Hard Drive Emulation
There are many benefits of using a virtual hard drive over the real thing, the first one being speed. Since you're not running anything off your sd card anymore, it loads much faster. The second benefit is that you can have unlimited storage space for your games which means that once they are installed onto the vhd file, all you have to do is change the name to open up another game without having to switch virtual HDD files every time. The third and final benefit is that this also works with netplay so installing your ROMs via link cable or other methods would make them run faster as well.
To conclude, an emulator that uses a virtual hard drive for its games is much faster and efficient than the real thing since it's running off your computer/pc rather than from your sd card. In my experience, games run anywhere from 3-6 times faster when using virtual HDD emulation as opposed to any other method of getting ROMs onto your machine. Of course, the process of getting your games to be compatible with a virtual hard drive is a bit more complex than just dumping your ROMs onto your sd card and loading them up normally but it's well worth it in my opinion.