My hard drive seems to run slower with a CD-ROM connected to the same cable.

While it may seem the hard drive has slowed, it is more likely that the entire system has slowed due to additional demand placed on the CPU by the ATA bus. The ATA interface uses the CPU for all transfer operations. Additionally, it is common for electronic timing conflicts to develop between devices using different levels of technology. This apparent slow down may be more pronounced with older, non-Direct Memory Access (DMA) systems and today's Ultra ATA capable hard drives. In accordance with the ATA Specification, an ATAPI device can coexist on the same cable as a hard drive, with each operating at their respective maximum transfer rates. However, experience has shown that, to avoid timing conflicts, it is always a good practice to place hard drives and ATAPI devices, such as CD-ROM and tape drives, on separate ATA channels when possible.

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