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Apr 3, 2003

Had my friend round look at my new system and we opened up the case. He looking inside then said that my Western Digital SE with 8mb Cache hard drive was Master and the IBM Deskstar (forget the name but it is a few years old now and its max is DMA66) was slowlying down my WDSE.

Is that true? Can the slower hard drive as Primary Slave slow down my faster one at Primary Master?
Last edited:


Very kind Senior
Mar 13, 2002
Greensboro NC
The answer is both yes and no. It can slow down the communication between the master drive and the memory, only while both devices are being used on the channel. The channel can only be used by a single device at a time, so one will wait while the other transmits or recieves. As to forcing the UDMA mode down, modern motherboards have dual fifo bufers and allow communication with each device at it's highest supported speed, as registered in BIOS at boot. Three things can change this. First, if a PIO mode only device is attached to the channel, the entire channel reverts to PIO mode, usually 16MB/s. Second, the motherboard doesn't support as high a mode as the drive has, it'll revert to the hightest supported mode. Finally, the OS(XP only at this time in released OS) can ramp down the UDMA mode in the event of CRC errors on the channel.

One of the more common fallacies on IDE is that the controller is on the motherboard. There is a controller on the board, but it acts more as a multiplexer to allow communication with the actual controller, which is mounted on the drive. The addressing is controlled by a signal on one of the cables. 0 is the master and 1 is the slave drive. Due to the addressing system and location of the controller, this is why different speeds are supported. The firmware on each drive is accessed by BIOS on boot and tells the BIOS what modes are supported. I hope this clarifies why I gave a longer answer.

It's best to separate all IDE drives on different channels to avoid contention for the channel, but sometimes not practical.

And Welcome to the forums!