Re-using a CD-RW for backup

Q: The first time I backed up to a CD-RW disk it worked fine.  Now when I try it I get an error message.  Isn't it supposed to be able to re-use the CD?

A: You need to either enter a different file name each time you make a backup to the same CD, or else re-format the CD (through Windows).  Unlike floppies or USB flash memory drives, it's generally not possible to overwrite an existing file on a CD (with the same name) without reformatting it first.

If using a different name doesn't help, it might also be that Windows is configured to "Close" the disk each time, preventing further re-writes.  Again, re-formatting the CD should work.