how to mount your virtual SD Drive automatically at system startup:
1. create your virtual SD file as shown above (in my case under Win7 I have a 12GB file located here: C:\ProgramData\IMDisk\virtual_sd.img )
2. create a text file with content <imdisk -a -f C:\ProgramData\IMDisk\virtual_sd.img -o rem -m Y:> and save this as start_SD.bat
3. copy and paste a link to start_SD.bat into you Autostart folder
your system will now automatically mount the virtual SD drive with letter Y at each startup
enjoy