I have arduino leonardo and sd card reader for arduino. What i wish to achieve, is to create an SD card reader for my PC. As arduino leonardo can act as USB device, so i thought it might be possible.
So you want to duplicate this?
The read speed is very slow. You won't be excited to see this arduino SD card reading working.
Everyone here - or very close to everyone - does suff that is reinventing the wheel. Not to be more efficient or save money but "because". Same reason why people climb mountains. Applaus and kudos to all inqusitive and tinkering minds !
Now, I do not have an Leonardo myself, but if i remember it acts (or can act) as a keyboard or mouse, not as a disk drive. That may the real challenge.
i am unable to open the link provided by CrossRoads.
This link is does the work, but is not for arduino. 
So can anyone see above link, and replicate it for arduino leonardo.