I had a policy on my blackberry 8800 (second hand from ebay) that prevented programs like mobipocket accessing the micro sd card even though I had replaced the policy with a blank one. Installing 4.5 certainly has fixed that. I didn't have to change anything, the restrictions just vanished.
I use a san disc 2gb card which is what I"ve seen recommended quite a lot. All files on it (from my old 4.2 install) work fine.
I did have trouble playing videos but after I deleted xplayer 4.2 the native video player started working perfectly and includes full screen view.
Since installing 4.5 I downloaded Cortado connect and that isn't working but I suspect some programes have not caught up with the new os yet.