SD card *mount/access* switching

[DBB2k12]

When I plug my phone in to a computer, I have the "Charge only - Charge phone over USB" and "Disk drive - mount as disk drive" options. I am extremely interested in finding out how my EVO switches access to my SD card from the phone itself to my USB connected computer and back. More so, I wonder if there's a way I can make an SD card in a laptop function similarly, using either a Windows or Linux OS [Windows is preferable but it seems like Andriod is Linux-based]. I just want to be able to take my second HDD/SD card/flash drive, mount it to a USB port, connect my computer to another via USB cable and be able to access the files on the second connected computer.

I don't know if this is beyond useful knowledge but I would be extremely grateful for any help at all as this would help me with a project I am very eager to begin. I'd also be glad to give a shout out/credit anyone that can help me on my new website.

Thank you!

 

[DBB2k12]

Apps like "Multi Mount SD-Card" and "Dual Mount SD" offer not exact but similar functionality. Really, I don't need the card/device mounted in multiple environments, I just need to be able to share it via USB cable from my laptop. Does anyone know of a way to do this without a USB Bridging Chip or other hardware? Virtual mounting would be ideal.