In recent years, traditional HDDs' areal density is reaching its limit. To extend the capacity of disk drives, several new storage techniques were proposed, including shingled magnetic recording (SMR), which is the first one to reach market among those new technologies. However, the shingled track structure of SMR disks encounters serious write amplification and performance declining when processing random write requests. Furthermore, constructing RAID5 based on SMR drives worsens the write amplification (WA) because the parity updating of RAID5 is very frequent to produce many random writes. This study, for current SMR disks' structure, finds that the first track of each band can be overwritten without impacting other tracks, because the wide write head can be moved a bit to cover both the first track and the guard region. In other words, the first track of each band can be called the free track, because it can be overwritten freely without causing any write amplification. Therefore, a new free-track-based RAID system (FT-RAID) is propose based on SMR drives, to fully develop the potentials of the overwriting-free region in SMR disk drives. FT-RAID is consisted of two key techniques, i.e., FT-Mapping and FT-Buffer. FT-Mapping is an SMR-friendly data mapping manner in RAID, which maps the frequently updated parity blocks to the free tracks; FT-Buffer adopts an SMR-friendly two-layer cache structures, in which the upper level can support in-place updating for hot blocks and the lower level can supply higher capacity for the write buffer. Both of them are designed to mitigate the degradation of performance by reducing SMR WA, leading to an 80.4% lower WA ratio than CMR-based RAID5 based on practical enterprise I/O workloads.