PIF's in blocks are most usually seen by interference into the IDE interface of the BenQ products.
A less-noisy power supply can help.
A new, flat 80 wire cable can help.
Setting the BenQ's IDE channel in BIOS to auto detect, auto type is really helpful.
And the #1 cause is UDMA mode 2 not completely implemented. The drive could be operating in Multi-Word DMA 2 instead. Another drive on the same cable could be adding to interference.
On CD/DVD speed, the PIF pattern of Multi-Word DMA2 looks a bit like
oo_o_oIo_oo_IIoooIIIoIIIIo where o = lots of little fat PIF blocks and I = normal PIF pattern. And, your scan has exactly that pattern.