A data backup method is provided. The data backup method includes maintaining a logical-to-physical table (L2P table) in a memory, where the L2P table records a plurality of physical addresses respectively corresponding to a plurality of first logical addresses and a plurality of trim marks; receiving a trim command, wherein the trim command indicates that data stored in one or more second logical addresses is invalid; according to the trim command, not clearing physical addresses respectively corresponding to the second logical addresses in the L2P table, and changing trim marks respectively corresponding to the second logical addresses from a first bit value to a second bit value; and when a predetermined condition is met, updating the L2P table according to the trim marks, and storing the updated L2P table into a rewritable non-volatile memory module.