Welcome to the wonderful world of flashcarts.
I play all my GBA games in TempGBA on a DStwo in a DSi XL. Been working very well for over a year. I got tired of polishing contacts and worrying about carts slipping in the middle of a game on my GBA SP.
For a while I used R4 "Revolution for DS" in DS Lite: very cheap 2GB max. SD, very nice Wood UI. Always worked very well in DS Lite, plays nice with 3-in-1 in slot-2.
On GBA (SP) I've had a lot of problems with flakiness with
EZ Flash IV, both the hardware and the software. (ROM patching just sucks and I was never sure I had the newest/best version of the utility.) Anyway, I got EZ-IV from the now defunct RealHotStuff and it works pretty well, but
my first one never read ANY SD card reliably and I'm probably going to sell/trade it to someone with better eyes and a steady soldering hand.
You could also try a
Radica Gamester Game Changer if you still use real carts in GBA SP: it lets you put 3 carts in at a time. It attaches VERY securely to the GBA SP. (The SP goes in the Gamester and the SP is secure on both sides, with its own weight even helping to secure the connection to the gamester (see picture linked), unless you play hanging upside down like a bat!) If you only use 2 of the 3 slots, it is also more comfortable to hold the GBA SP. The only problem is that EZ-Flash IV will not work in it and the third cart sticks out awkwardly.