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Lew 09-16-2008 11:06 AM

GUID vs. Apple Partition Map?
I have read here on the forum that external drives need to be partitioned using the GUID scheme in order for them to be bootable.

I have two 1TB LaCie Big Disk Extreme+ drives. One is used exclusively for Time Machine, the other is partitioned using Apple Partition Map into three drives: one for data, the other two are for use by SuperDuper! for alternating nightly scheduled backups.

I was concerned after reading about the necessity of partitioning using GUID, so I booted my Macbook Pro while holding down the option key. I was able to select one of the two drives with SuperDuper! backups and booted up with no problem at all.

My questions:
  • Am I missing something here?
  • Should I repartition my Lacie using GUID?
  • What is the advantage of GUID over Apple Partition Map?
BTW, I'm running OS X 10.5.5.



dnanian 09-16-2008 11:11 AM

Apple strongly encourages GUID use with Intel (not Power PC) Macs, to the point that they suppress the listing of APM 'bootable volumes' in the startup disk preference pane.

It's hard to know what the advantages are or aren't -- but it's pretty clear that Apple wants GUID used.

Lew 09-16-2008 04:01 PM

Thanks, Dave. I guess I'll bite the bullet and repartition my drives to GUID.

BTW, the two APM partions that SuperDuper! backups reside on do appear in my startup disk preference pane.


dnanian 09-16-2008 04:30 PM

Are you on an Intel Mac?

Lew 09-16-2008 04:36 PM


dnanian 09-16-2008 04:37 PM

Perhaps they've loosened this requirement in 10.5.5, Lew; they were certainly suppressed before. Regardless, though, they do strongly encourage GUID use, even though APM works.

RonaldPR 09-16-2008 04:56 PM

When you do a new OS installation on an Intel Mac, only GUID Partition Table volumes appear as target for the installation. However, Intel Macs do (still) boot from Apple Partition Map volumes. I wouldn't be surprised if this proves to be a transitional situation that will change with OSX 10.6.

rlfsoso 09-17-2008 12:36 PM

Advantages of GUID-Scheme

to sweeten the deal: with GUID-scheme it is possible to change/adjust the sizes of partitions "live" without reformatting the drive with Drive Utillity. Of course this only possible if you have free space on the partitions.



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