From O'Reilly: PC Hardware in Nutshell 3rd Edition

Always assign the main hard drive as Primary Master. Particularly if your hard drive supports fast DMA modes (ATA-66 or ATA-100), do not connect another device to the primary ATA interface unless both positions on the secondary interface are occupied.
最好把主磁設成 Primary Master, 可以的話 Primary 不要再接其它東西。


ATA forbids simultaneous I/O on an interface, which means that only one device can be active at a time. If one device is reading or writing, the other device cannot read or write until the active device yields the channel. The implication of this is that if you have two devices that need to perform simultaneous I/O—e.g., a CD writer that you use to duplicate CDs from a CD-ROM drive—you should place those two devices on separate interfaces.
就是在說對燒的機器不要裝在同一條排線上。

If you are connecting an ATA device (a hard drive) and an ATAPI device (e.g., an optical drive or a tape drive) to the same interface, set the hard drive as Master and the ATAPI device as Slave.
硬碟和光碟可以接在一起，但要把硬碟設成 Master, 沒有說到慢的裝置會拖慢快的裝置。

If you are connecting two similar devices (ATA or ATAPI) to an interface, it generally doesn't matter which device is Master and which Slave. There are exceptions to this, however, particularly with ATAPI devices, some of which really want to be Master (or Slave) depending on which other ATAPI device is connected to the channel.
同樣類型的裝置接在一起，比較沒有誰設 master 的問題


If you are connecting an older device and a newer device to the same ATA interface, it's generally better to configure the newer device as Master because it is likely to have a more capable controller than the older device.
新的裝置設 Master 比較好，但沒有提到舊的會拖慢新的。

Try to avoid sharing one interface between a DMA-capable device and a PIO-only device. If both devices on an interface are DMA-capable, both use DMA. If only one device is DMA-capable, both devices are forced to use PIO, which reduces performance and increases CPU utilization dramatically. Similarly, if both devices are DMA-capable, but at different levels, the more capable device is forced to use the slower DMA mode.
DMA 不要和 PIO 接在一起，不然會被強迫兩個都用 PIO

13.1.6 ATA Cables