My burner is a Yamaha 2100SCSI with 8MB buffer; the HDD is IDE. Sometimes I can burn at 16x but anything over 8x has a 1 in 6 failure rate due to buffer underrun even though I think I've got nothing running. But if FindFast or Norton or any daemon kicks in on the vinegar stroke and then uses the HDD, you'll more likely get a buffer underrun in proportion to a combination of higher speed burn as handicapped by a fragmented HDD.

EIDE in the Primary/Secondary HDD/Burner relationship with DMA set for the HDD should perform as per SCSI. Windows is the bugger!