A good program to check how much space is left on a partition is "Conky" or even better "Storage Usage"

Music/Videos/Images not found
1. Check your home partition (2GB) and your rootfs (256 MB) and see that they are not full.
2. Use tracker-cfg to edit paths where media is searched and restart the tracker. Please be careful when editing things here, however it has revert to default for safety.

Applications not installing for missing dependencies
Try disabling extras-devel. Let it update the repositories and if you find the same app (it will be in extras), it will most probably install.

How to 'enable' hot-swapable microsd cards
Not a fix in itself. This is unknown to most people. If you take off the back cover, the microsd will not be displayed in the file manager because there is a sensor (magnet) that knows when the back cover is on. This means you can remove and replace your microsd cards at will without taking out the battery.

Catalogues not displayed? SMS cannot be saved because 'Device is full'? N900 not working properly
Check your rootfs (256 MB) partition to see if it is full. Search for 'optifying n900' in the wiki pages to learn how to free it a bit.