Windows 7 backup

Possible Duplicate:
Does built in Windows 7 backup perform differential (incremental) backups?

Hi,

I use Windows 7 Backup for simple home backup. Windows 7 backup seems to maintain about 3 versions of the backup, for the last three days. I only ever want the lastest set of files to be backed up - is that an incremental? I basically only want it to backup those files that have changed.

I know that its better practice to have multiple versions of the backup, however I swap out external hard drives every couple of days, so I do always have a couple of versions.

Thanks Al