One of the things I like about the Android OS

Is that when things go wrong, the answer is only a Google search away.

For example, my applications stopped updating to the SD card on my phone. I did a search and the thing that worked was this:

Connect your phone through the USB, click on the folder /sdcard/.android_secure and there's a temporary file called smdl2tmp1.asec. Delete that file (it's safe because it's a temporary file) and you should be able to download and install updates again.

This idea was courtesy of Alex_I on the Android forums.

It worked like a charm for me. :)