Don't use PastaCFW, it needs CubicNinja. Rx tool provides the same without need of additional stuff. If you use RXtools you can keep everything on the same SD card.
Instructions, it's very VERY easy.
-Go to the thread "RX tools is out with signature patch"
-Download the version wiyh twl and agb patched (Ssb and MH4 won't work)(no worries, you have GW mode for that )
- place the downloaded file in your SD card
-Go to the original RXtools threat (or site) and look for the program that provides firmware. bin. *an easy one click app that provides this file.
-put it on the root of your sd card
-Go to gateway mode and install the AGB patch. (just follow the AGB patch threat. )
-after the installation of the AGB patch and GBA games, shut down your 3DS.
-just turn it on again
-Open the browser on your 3DS and enter the launch site for RXtools (
http://launch.rxtools.net/rxTools)
-Just choose launch Rxtools
-Now you're in RX mode in your emunand (profit)
note:if you installed the AGB patch correctly and you launch your gba game in RXMODE and you STILL got a black screen.
Try to use/make another GBA cia. If they're having a bad footer, they provides black screens or errors.