Make sure your SD card isn't full. Downloads are saved to a folder on the SD card called download. Also (though unlikely) make sure your SD card isn't in removal mode.
Check free space:
Settings / SD Card & Phone Storage / SD card Available Space
Also under Available Space the Unmount SD Card option should be bold and Format SD Card should be grayed out. If they are reversed, do not press Format SD Card, simply reboot your Eris and this will correct the SD Card removal mode issue.