One of my Christmas presents was a 2GB microSD card for my T-Mobile Wing phone (a very nice Windows Mobile 6 device, extremely happy with it). I thought it would be nice to sync some music to it so I could use it as a player. So I attached it to my Windows Vista PC, went into Windows Media Player, and went to the Sync tab. So far so good: it sees the device. But it doesn't see the storage card (even though Explorer does), and the internal storage on the phone can't store more than a few songs.
After swearing at Microsoft, I remembered that I work for Microsoft - makes the swearing kind of pointless :-) So I started searching for a fix, which led me to this KB article:
One reboot later and problem solved. One of these days Windows will proactively tell me about these types of things...