Ask Slashdot: Asynchronous RAID-1 Free Software Backup For Laptops?

First time accepted submitter ormembar writes "I have a laptop with a 1 TB hard disk. I use rsync to perform my backups (hopefully quite regularly) on an external 1 TB hard disk. But, with such a large hard disk, it takes quite some time to perform backups because rsync scans the whole disk for updates (15 minutes in average). Does it exist somewhere a kind of asynchronous RAID-1 free software that would record in a journal all the changes that I perform on the disk and replay this journal later, when I plug my external hard disk on the laptop? I guess that it would be faster than usual backup solutions (rsync, unison, you name it) that scan the whole partitions every time. Do you feel the same annoyance when backing up laptops?"