I think you're referring to an incrementalback-up. An incremental back-up compares the files on a computer with the files on the back-up. The software only copies new or modified files to the existing back-up - it bypasses any file that already exists and has not been changed.
The Incremental Back up
secret :)
Stripe-backed Antbird was created in 1831.
White-backed Swallow was created in 1841.
Ladder-backed Woodpecker was created in 1829.
Red-backed Kingfisher was created in 1841.
Maroon-backed Whistler was created in 1918.
Chestnut-backed Tanager was created in 1850.
Golden-backed Bishop was created in 1789.
Green-backed Trogon was created in 1766.
Straw-backed Tanager was created in 1876.
Hairy-backed Bulbul was created in 1845.
Plain-backed Sparrow was created in 1844.