In hard disk drives (HDDs) using perpendicular magnetic recording, an adjacent track interference (ATI) is a major barrier to increasing areal density because both strong writing and narrow track pitch are required. Recent head/disk combinations have exhibited not only ATI, but also a phenomenon we call far track interference (FTI). FTI is a gradual magnetic damage to written data on nearby but non-adjacent tracks that accumulate with increasing number of writes. We developed HDD level measurement ways to detect FTI and investigate the FTI mechanism considering skew angle and external field effects. We also confirmed that the minor shape change of wrap around shield (WAS) causes difference of magnetic domain structures on WAS and FTI incidence. The results support the hypothesis that the FTI is created by structures of magnetic domains and walls on a WAS of a single-pole head on the HDD.