AVR with SD as mass storage device?

Is it possibly to have an AVR like the 32U4 coupled with a micro SD card, to show up on a computer as a mass storage device, so you could just copy files back and forth?

If so, can someone point me to info on how to achieve that?

So you want to have Arduino do what this does? http://www.bestbuy.com/site/Dynex%26%23153%3B+Direct+-+Direct+USB+2.0+Secure+Digital+Memory+Card+Reader/3143941.p?id=1218379769422&skuId=3143941

If you still want Arduino in the middle, then check out fat16lib's library for Arduino http://code.google.com/p/fat16lib/

Sort of. Basically I want to be able to plug in the device and a) be able to download new code to it, or b) have it mount the SD card as a mass storage so I can copy a file (or more) to or from it

It's a similar setup as what I see some mobile phones are able to do. When I plug my Nexus in, it automatically comes up as a mass storage device. When I enable USB debugging on the phone, then I can also communicate with it via ADB and install software or whatever (and it's also still mounted as a mass storage device.)

I'm not sure I can do that with a 32U4 though, at least with the limited reading I did, I may not be able to. Seems like I may have to push up to a 32UC3 device.

Yeah, so you want to do what this eval board is doing.
Or, incorporate a USB card reader chip, and toggle back forth between the PC having control of the SD card, or the Atmega having control of the card.
There’s a chip I found that does PC/USB interfacing to SD cards, I’ll see if I can find it again (I have its datasheet someplace else).

USB-SDcard.pdf (1.7 MB)

Yeah, I found that too in my searching, which let me to believe I would need to go to a 32UC3 uC.

KirAsh4: Is it possibly to have an AVR like the 32U4 coupled with a micro SD card, to show up on a computer as a mass storage device, so you could just copy files back and forth?

That's something I was looking at some time ago and found this link useful (http://we.easyelectronics.ru/AVR/usb-fleshka-na-atmega8-i-v-usb-ot-idei-k-gotovomu-ustroystvu.html) but hit a wall trying to massage the LUFA libraries and the code to arduino land.