Format it on your pc first, see what I gets you. If its ok(~8gb), then install again and check it. If its ok again then have your bb format it anyways and see if it gets you some extra space, mine did. If not more info may help.
Also, what make/model of card is it?
Posted from my CrackBerry at wapforums.crackberry.com
It came with an adapter but I don't have a slot on my computer for the adapter, it wasn't a usb adapter. I also just upgraded to 4.5 because AT&T actually released it finally.
For some reason, there have been a lot of threads lately relating to problems with the media cards not being recognized, unable to format, etc. on OS 4.5
Sorry was gone for a while. In response to the last poster, why would would I take it back. It seems more so like something is wrong maybe with the phone and reading it. When I did go to format it the card it showed that it had 3.8 gb already but when i formatted it didn't change. Even though 4.5 is suppose to be able to read up to 8gb of memory, maybe there is something wrong with the update AT&T released?
I use a 8GB sandisk micro SDHC in my 8310. When I first got it my OS was 4.2 and only recognised 3.7GB I think, however since upgrading to 4.5 it now displays 7.5GB.
I did initally insert it in to my PC via the USB adapter it came with and formatted it with that though. I did this again once I upgraded to v4.5 aswell due to having been using it for something else at the time and wanting to wipe it.