I frequently try new ROMs for my Windows Mobile device. On my last ROM I came across an issue that prevented me from storing Email attachments on my 2GB microSD storage card. The Storage error says, "No unlocked storage card detected. Make sure an unlocked storage card is inserted and try again."
The problem occurs when Windows Mobile thinks the extended ROM is your storage card, and the extended ROM is locked (the default behavior).
Notice in the screenshot above that WM6 says I have only 8.52MB free (the extended ROM), not 2GB as it would be if it were looking at the storage card.
The fix is to hide the extended ROM, restart the mobile device and reconfigure email to store attachments on the storage card. You can then re-show the extended ROM if necessary.