It seems like either the drive letter may have been changed or you do not have the correct removable media connected that you set your backup to be saved to when it runs automatically on a schedule. When the backup runs, it seems that it can't find the drive that you set it to be saved to, and giving you this error.
What is the full message when you click on the Insert removable media (Important)
link from the notification area on your taskbar?
What does it say when you click on the Options
Do you have the drive that you set to save your backups to connected, and turned on if it's an external HDD?
Look in your Backup and Restore
window like below to see what drive letter it is set to save the backup to, then check to see if the drive letter for the drive you have connected to save it to is the same or not. For example, mine is set for my Test D: