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Synchronous writing in BPMR has been recognized as a crucial yet challenging issue. It has been shown that position jitter and switching field distribution can lead to spatially uncorrelated random write failures. On top of that, the spindle speed variation and other mechanical vibration may lead to accumulative phase drift during writing which causes long streams of insertion/deletion write failures. However no experiments have been conducted to quantitatively testify the later concept, nor provide precise written-in error characteristics when both phenomena are present in BPMR systems. This gap of understanding is filled by this work through spinstand and hard disk measurements and analysis. It is shown that timing inaccuracy not only introduces insertion/deletion write failures but also given rise to substantial increase of substitution (random) write errors. It also reveals that instead of the commonly accused spindle motor speed variation, timing error in BPMR based HDD may well be the result of estimation error due to limited or improperly configured timing preambles.