PRAM reset may result in wrong boot drive being used

"PRAM" (Parameter RAM) consists of a small chip of memory, which contains hardware parameters that the machine can use before the system is loaded. This information includes system volume, screen settings, as well as the preferred boot device. Depending on which drives the operating system is installed, users can run into small problems where after resetting the PRAM the system will either pause before booting, or even boot to the wrong boot drive if multiple operating systems are present.

Apple Discussions poster Carlton Chin:

"I have Mac OS Leopard (10.5.6) installed on my primary startup drive (500GB). I have Tiger (10.4.11) installed on the second drive (300GB). Both drives are internal. Normally when I restart my G5 it boots into Leopard, which is correct. Why is it that whenever I reset-nvram and reset-all in Open Firmware it boots to the second drive that has Tiger? "

When users reset the PRAM, the partition to boot from will then be set to the first partition on the first drive presented to the system by the hard drive controllers. As with most PCs, hard drives are initialized in sequence based on their connections, which means if a purely storage drive is first in line, it will be set to be the "boot" drive after a PRAM reset. As such, without a designated boot drive the system will scan through all available partitions and drives until a viable boot drive is found. Meanwhile, users may see a blinking question mark appear on the screen or see the computer boot to a secondary OS installation by default.

While the easiest solution to this problem is to use the "Startup Disk" system preferences to set the desired boot partition, for users who frequently reset the PRAM this can be prevented by ensuring their preferred operating system is on a drive or partition that's before any other drive containing a bootable system.

In many instances users will have two drives that already contain system and user files, and formatting and reinstalling the OS may not be preferred. In addition, users might have one performance drive for booting and another slower one for data storage, so reinstalling the OS in this case might also not be preferred. In these instances, the easiest solution is to swap the drives in their bays so the preferred boot drive is always accessed first unless specifically set otherwise in the "Startup Disk" system preferences.

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