I have been backing up my macbook on a passport external hard drive. Now the backups are "failed" because there isn't enough room on my passport, can I set it up to override the previous backup and only keep one file instead of a separate one everytime?
If your backup software was doing "incremental" backups then they would eventually consume all available space on the drive. Check the backup application and perhaps change it to full backups / overwite previous.
You may have to manually remove the incrementals, but I dont know how your would do that on a MAC, just a PC guy.