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Staxman 07-21-2009 12:28 AM

Partition table and bootability
 
I have a G4 iMac running OS X 10.4.11. For ~2 years I've been cloning my internal HD to an Other World Computing Mercury On-the-Go drive:

http://eshop.macsales.com/shop/firewire/on-the-go

A few months ago I picked up a Verbatim external drive from one of the online vendors (Buy.com, Newegg.com, I forget who). I started cloning my drive to the Verbatim drive also. Both external drives are enough bigger than my internal HD that I have multiple partitions on each, so I have quite a few clones available at any given time. I partitioned each drive at the time I got it.

Long story short, some recent events (and a discussion on another Mac forum) led me to take a closer look at the partitioning schemes on the drives. I found that the OWC drive was partitioned with Apple Partition Map (not surprising, coming from OWC), and the Verbatim drive was partitioned with Master Boot Record.

Whenever I've made a clone, the Mac has seemed to reboot successfully from the external drive, and according to System Preferences, the external drive has been set as the boot drive. Once the computer was up and running, I've gone into System Preferences, set the boot drive to my internal drive, and rebooted.

According to the discussion on the other forum, my Mac shouldn't have been able to boot from the Verbatim drive. Just as a test, I plugged in the Verbatim drive and depressed Option as the system was coming up. I was offered only the internal drive as a possible boot drive. This leaves me wondering what's really been going on when I've thought I was booting from the Verbatim drive. If I can't boot from the Verbatim drive, shouldn't the system be telling me so?

This also seems odd to me because about 3 years ago I had an experience (which I posted about here at the time) with an external drive whose chipset didn't support booting. In that case, the computer was clearly not booting from the external drive. I wound up using that for offline storage and got the OWC drive.

I realize that I could boot the system from the OS X install DVD and restore from the Verbatim drive, but I think that defeats the purpose of using SuperDuper. If I have to repartition the Verbatim drive with Apple Partition Map, so be it.

Thanks.

chris_johnsen 07-21-2009 03:37 AM

I have never heard of a Mac booting off of an drive with solely an MBR partition (discounting the GPT/MBR hybrid that Boot Camp can create), but I suppose it is possible (after all, Intel Macs reportedly can boot from APM, it is just not officially supported/advertised).

Do you usually have both external drives connected to the system? Are the volumes on both exernal drives named similarly? Maybe you misread it as booting from a volume on the Verbatim but it was actually booting from the OWC.

When determining the startup disk for the computer as it is currently running, you should always look at Startup Disk in  > About This Mac.

Are you using the "Restart from …" or the "Set … as Startup Disk" options in SuperDuper!? Maybe those options uses bless in a way that can configure the startup disk to use a partition that System Preference's Startup Disk preference pane and the boot-menu (boot + Option) will not allow the user to choose (it is fairly common for the GUI to offer fewer options than the underlying CLI/API actually supports).

It could be that the Open Firmware was actually configured to boot from a partition on the MBR Verbatim disk. This is speculation, but in such a situation, the firmware might notice that it can not possibly startup from that volume. Faced with this "insane" configuration, it might try to boot from a "normal" volume (like one on the internal disk). That would cause System Preference's Startup Disk to show that a particular volume is selected while the system consistently boots from some other volume. This is another reason to use About This Mac to check the actual startup disk. Such a hypothetical "boot-time decision" by the firmware or even a normal Option-boot by the user can make the actual startup disk different from the configured startup disk.

Assuming the Verbatim disk is otherwise bootable (good chipset), you will probably have to repartition it as APM before any of its volumes will work reliably as the startup disk. If you can not stand to erase the whole disk (what Disk Utility will do), you could look at iPartition.

dnanian 07-21-2009 08:28 AM

I have no idea how a Power PC Mac could have worked with an MBR partitioned drive. In a quick test here it certainly doesn't!

Staxman 07-21-2009 10:19 PM

Partition table and bootability
 
Quote:

Originally Posted by chris_johnsen (Post 25702)
Do you usually have both external drives connected to the system? Are the volumes on both exernal drives named similarly? Maybe you misread it as booting from a volume on the Verbatim but it was actually booting from the OWC.

No, I've only ever had one external connected to the system at a given time. Even if that weren't true, all the partitions have distinctive names.

Quote:

Originally Posted by chris_johnsen (Post 25702)
When determining the startup disk for the computer as it is currently running, you should always look at Startup Disk in  > About This Mac.

I didn't know that--live and learn!

Quote:

Originally Posted by chris_johnsen (Post 25702)
Are you using the "Restart from …" or the "Set … as Startup Disk" options in SuperDuper!?

I don't know--next time I run SuperDuper, I'll check. That'll probably be this weekend. The drives live in my safe deposit box except when I'm using them.

Quote:

Originally Posted by chris_johnsen (Post 25702)
could be that the Open Firmware was actually configured to boot from a partition on the MBR Verbatim disk. This is speculation, but in such a situation, the firmware might notice that it can not possibly startup from that volume. Faced with this "insane" configuration, it might try to boot from a "normal" volume (like one on the internal disk).

Strikes me as the most likely scenario. This all came about because I got a WinBlows box a while back for work-related software and have been backing it up to an NTSF partition on the MBR disk. I wanted to create a Windows partition on the APM disk, and when I found I couldn't, that led to the discussion on the other forum. I was hoping to have data from both computers on both drives in case one drive died. Sounds like I'm going to have to go all-Windows on the MBR disk. or accept that I'll need to boot from the install DVD to restore from the Mac partitions on that disk.

Thanks for a good, informative explanation--really clears things up for me!

Staxman 07-31-2009 11:50 PM

Boot disk
 
Quote:

Originally Posted by chris_johnsen (Post 25702)
When determining the startup disk for the computer as it is currently running, you should always look at Startup Disk in  > About This Mac.

This may sound like a dumb question, but where in About This Mac is this information? I found it last week, but right now I must be blind.

chris_johnsen 08-01-2009 12:58 AM

In my About This Mac window it is right above the "More Info…" button. Besides OS version beneath the prominent "Mac OS X" text, there are two buttons and three lines of text info: "Software Update…" button, "Processor" info , "Memory" info and "Startup Disk" info, then the "More Info…" button.

Staxman 08-01-2009 08:35 AM

About this Mac
 
Quote:

Originally Posted by chris_johnsen (Post 25847)
In my About This Mac window it is right above the "More Info…" button. Besides OS version beneath the prominent "Mac OS X" text, there are two buttons and three lines of text info: "Software Update…" button, "Processor" info , "Memory" info and "Startup Disk" info, then the "More Info…" button.

Same here! I was so eager to find the info that I blew right past it! Talk about hiding in plain sight! Thanks.


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