Invisible sd card
From Hobbyist Software
It seems that sometimes adding an extra app to your Treo can cause it to stop recognising your SD card. My theory is that there is some timer running on the Treo that tries to mount your SD card x seconds after a reset starts processing. If you have too many apps doing too much (like registering themselves for notifications) then the timer doesn't trigger and the SD card doesn't get mounted.
Just install cardkeeper, then select the 'remount' option and all should be fine...
Other users have had success using uncache to reduce the number of applications which are called on startup.