Just a bit confused about external drive partitioning

Discussion in 'Mac Basics and Help' started by pearlyvictoria, Mar 11, 2011.

  1. pearlyvictoria macrumors newbie

    Jul 26, 2010
    Hello, I just bought an iMac a couple of months ago. Since then I've been using Carbonite to back up my files. In short, it doesn't back up all files so I am going to use Super Duper. Please bear with me while I explain my confusion :)

    I have an 1TB Verbatim external USB drive that I use with Time Machine but am unsure about how I partitioned it or when (with the old G-4 or with this new intel iMac.

    The information says it is Mac OS Extended Journaled. Super Duper recommends using a Guid format. I do not know if it is Guid or not so I downloaded iPartition to re-partition it but do not see a format option for Guid.

    I need to back up my files so I can re-install the OS from the disk because I stoopit-lee drug over an application file instead of re-installing when setting up my new computer. Needless to say it was corrupted.

    I am running:
    OS 10.6.6
    3.06 GHZ
    Intel core 2 duo
    4G 1067 MHz DDR3
  2. Makosuke macrumors 603

    Aug 15, 2001
    The Cool Part of CA, USA
    GUID is the partition map type, not the format of the data partition itself.

    You can read tutorials elsewhere, but in general terms the partition map is the very low level way in which the drive and its partitions (whether there is one or many) is identified to the OS. The available MacOS options are GUID (the newest, and current standard), APM (Apple Partition Map, the PowerPC era standard--this is probably what your drive uses), and MBR (Windows standard).

    Regardless of the partition map type used, individual partitions can be in any of a number of formats--MacOS Extended (aka HFS+), HFS+ Journaled and/or case sensitive, FAT, NTFS, ExFAT, etc. There can even be more than one type of partition on the same drive--for example, if you ran BootCamp, your Windows partition will be either FAT or NTFS, while your OSX partition is going to be HFS; in such a case the partition map will be GUID--BootCamp requires it.

    In your case, iPartition will not be of any use--to my knowledge you cannot change the Partition Map format on the fly without completely erasing the physical drive and destroying all partitions on it. So what you're going to need to do, if you want a GUID drive, is this:

    Back up everything on the drive (or, if it's the backup drive itself, take the risk and do a fresh backup as soon as you're done). Open Disk Utility and select the drive itself--not the partitions indented under it--in the left pane. You'll be able to see what the current Partition Map Scheme is in the info displayed at the far lower right corner of the window. It's probably APM in your case. To change it, go to the "Partition" tab at top (this will only be visible if you have the physical drive selected), select the partitions you want from the drop-down (probably "1 Partition" for your backup drive), then click the Options button--you'll get a window offering the three types of drive. Select GUID there, click OK, make sure that the partition itself is going to be formatted as "MacOS Extended (Journaled)", and finally click "Apply" at lower right.

    The drive will be completely erased, and you'll end up with the desired partition scheme and the partition format you want. At that point you can run Time Machine or whatever backup software you feel like.
  3. pearlyvictoria thread starter macrumors newbie

    Jul 26, 2010
    Thank you so much for the detailed information, which i am going to save. But I am a little dense on this still....

    You say to make sure the partition itself is going to be formatted as MacOS Extended (journaled). That what disk utility says it is already. Does that mean it is already a GUID partition?
  4. Makosuke macrumors 603

    Aug 15, 2001
    The Cool Part of CA, USA
    No. A MacOS Extended (Journaled) partition can exist on a GUID disk, or an APM disk. Since you mentioned that the drive dates to a G4, it is almost certainly using an APM map. Again, the partition map is what it sounds like--the map of whatever partitions are on the drive. The actual format of the partition(s) itself has to do with the file structure and features available.

    If you need a GUID disk to do what you want to do, you're going to need to completely repartition it to use GUID. That's repartition, not reformat, although they are sometimes used almost interchangeably. Reformatting an individual partition only changes the format of that specific partition--it has no effect on the underlying partition map. Look back over what I already wrote--I explain how to check what partition map scheme a particular physical drive is currently using.

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