I have an OWC 320 GB drive partitioned for backups (Super Duper & Time Machine). It contains a Seagate Momentus 7200.4 and is attached by Firewire 800.

If the drive is running, on a reboot my iMac does not boot from the internal drive. It boots instead from the OWC. Consequently I can't leave it running (which defeats the purpose of having Time Machine backups) so I just turn it on when I need a backup.

Sounds like some bit of software needs a tweak but what bit?

Check /Apps/Sys Prefs > Startup Disk and see what's selected.

Edit: On both the iMac and the external, I guess.

The "Startup Disk" setting is stored in PRAM aka NVRAM. It's independent of the drive. (It has to be. It's what selects the drive.)

On startup, if the selected boot volume can't be found, the boot logic will wait for it to appear, and then after a timeout take whatever volume is available. That means that even if turning off the external lets you boot from the internal, you're still slowing down startup by however long the boot logic waits for the selected drive to appear.

So the solution should be to reset PRAM?

NO!!! The solution is to go to System Preferences→Startup Disk and select the volume you want to boot from normally. (Which, I imagine, is your internal.)

Resetting PRAM would be like breaking a window instead of opening it, to get fresh air into the room. It'll get the job done, but now you have to replace the glass to reverse the operation.

Resetting PRAM discards all PRAM settings, including the one for "Startup Disk". If you have no selected startup disk, the boot logic will select what it thinks is the most likely (starting with the first partition of the first internal drive, and working systematically through the alternatives until it finds one that's actually bootable). But it won't start looking for the best until after waiting to give all drives time to spin up. Again, you're slowing down the boot process.

Thanks for clarifying that. (I was scratching my head as I wrote that edit, and under any circumstances, I should have immediately specified "on your iMac," rather than merely implied it.)

I've never been quite certain what determines Startup Disk; is there any way that booting from a volume, either on another partition or external, other than your regular boot volume can affect your Startup Disk selection in PRAM? (I think I managed to somehow change my selection a coupl'a weeks ago.)

Thanks.

I do not know of anything specific that will reset the boot volume setting stored in the PRAM other than System Preferences > Startup Disk and its Unix command equivalents. However, the setting is only a bit or two and an electronic hiccup could possibly flip that bit without your being aware of it. I have seen a couple of cases where the selected boot volume was either not present or had become non-bootable for some reason which forces the system to look for the next bootable volume in line. There is no warning or notice of any kind that the system is booting from a volume other than the selected boot volume. If the alternate boot volume is a clone the user may never notice which volume is booted. I always change the desktop image on cloned volumes for this specific reason.

Recognizibly different background images for main and cloned drives is good practice for several reasons. It's simple and cannot be promoted enough.

Merely booting from a different source (for example, by holding down the option key at startup) shouldn't permanently change the setting.

The PRAM is parity-checked, so random changes (such as by cosmic rays or some-such) should have the same effect as zapping PRAM. That is, it will set this setting back to the default, which is to prefer the internal.

The ways I know of to change the setting, in order of increasing geekiness, are:

• System Preferences→Startup Disk
• Running the system installer (which I believe will want to make the new system the default)
• A full restore from a TM backup (which will ask if you want to make the restored system the default)
• The bless command with the --setBoot option
• Booting into Open Firmware or EFI and setting boot-device or efi-boot-device as appropriate

Using a different background image won't help notice that you've accidentally booted into a clone, because a clone will have the same background image.

My main drive has a blue background on the desktop and the clone has a gray one. I can, therefore, tell immediately which one I'm booted from.

That would be correct if you leave things alone after cloning. But there's little reason to do that, and changing the desktop image of the main or the clone after cloning would produce the desired distinction.

How?

Thanks to you and joemike.

I don't know what happened, but all of a sudden I found myself booted into the wrong volume, and when I looked, I found that my Startup Disk selection had changed without my having changed it.

I wonder if it was maybe a QuickBoot glitch. (I use QB to avoid option-booting, and it never touched Startup Disk in the past, but who knows?)

Guess I'll never know.

Thanks for adding that. I should have stated that I change the main drive's desktop background before cloning and change it back afterwards.