Sometimes when we use a new custom rom, we might encounter a USB Mass Storage problem. As a result, the Mass Storage notification might not pop up and hence we can't access our SD Card. But fear not, for I have made a simple app to enable USB Mass Storage. Firstly, a rooted phone is a must. This app will not auto-start, so you'll have to start it whenever you want to enable USB Mass Storage. In this app, there are 2 buttons, Turn ON USB and Turn OFF USB. Always ensure that your USB cable is connected before clicking any of those buttons. Also it is always advisable to safely remove USB before clicking the Turn OFF USB button. I've personally tried this on my Samsung Galaxy Ace running AOSP Gingerbread for Cooper by @SpaceCaker and its working fine. If you come up with any problems, post it here. I'll be glad to help.
Note : I haven't tested this on any other device apart from my Samsung Galaxy Ace. You can always try it on any other device. If it does work on any other device, make sure to post your device in this thread
P.S. Don't forget to hit the Thanks button for all my hardwork. Also if you want to share this app, please share a link to this thread.