I am learning to use the Backup MyPC program. Why is the file size on my external hard drive I'm using for backups smaller than the amount of data on my C: drive on the computer, by about 3 GB? Also, whenever I choose to do an incremental or differential backup, it asks me if I want to overwrite the existing backup file or not. How should I answer that request? Thanks. dab

It is smaller, because it compresses the data. Sort of like a Winzip program compresses files so you have lesss overhead. If doing an incremental, you do not want to overwrite as that would destroy the previous incremental backup backup. You can overwrite the previous differential backup if doing that type as the differential is a backup of all files changed since the last full backup. Incremental is a backup of everything changed since the last full or incremental backup.
In no case would you want to overwrite the full backup, unless you are doing a full backup.
Also in your options, you can specify from the start so it will not ask you whether to ask, always overwrite or always append.