presale advice

It depends on whether your backup program is file-based or block-based. File-based backups which is more common, will copy the entire file regardless of whether one byte or the whole file changes. Block-based backup will need to determine which blocks changed and will copy just those blocks, however depending on the speed of your computer and how much memory it has, it may be faster copying the entire file rather than searching for the differences.

That said, even with file-based backups, it'll typically see if there's any difference in the file before copying, so it is incremental in that only the changed files will be copied. The ReadyNAS comes with licenses for Memeo Premium Backup which will do the file-based incremental backups to the ReadyNAS any time the file changes. It's also smart enough to back off if there's an intensive foreground task and will stay in the background waiting for a good window to continue. That, or if you have a backup program with scheduling, you can setup your backup jobs at off-hours.