Separate Downloads folder on SD Card
Please explain your question better. Are you trying to move your User "Downloads" folder to another location than the harddrive? Are you wanting to have a separate folder you download to moved to an SDCard?

You can easily create a new Downloads folder on the SDCard and when you download files just save there by default. I could do some searching (or you can) to see if there is a way to have a different folder open by default to save, but it's usually the last place you saved a file to.

If you are wanting to MOVE your Downloads folder, you can always create some sort of symbolic link where your current Downloads folder is and have it point to the SDCard, the problem with this is that if you SDCard EVER drops away you have a potential to have to recreate the link when card is seen again. There is no such thing as a "permanent" installed removable media.

For further reference, THIS LINK pulls up as a first search on google and walks you through the various options I mention above.