Need to increase the current size of my Time Machine partition but Disk Utility doesn't allow me to

Discussion in 'macOS' started by iRock1, Jan 14, 2016.

  1. iRock1 macrumors 6502a


    Apr 23, 2011

    I'm using an iMac with a 500 GB SSD and 1 TB hard drive, and also an external storage device, which is configured in RAID5 with 8 usable TB (I split it in two partitions: one of 6 TB for my media and a Time Machine partition of 2 TB).

    The problem is that my Time Machine partition ran out of space, as it has to hold the contents of both my SSD and secondary hard drive. Because of this, the system is constantly reminding me of the fact that Time Machine is running out of space and that it needs to delete old backups.

    Right now the 6 TB media partition has 1.3 TB free. So I'd like to take some of that space for some time, and using Disk Utility reassign it to the Time Machine partition, just like you can increase or decrease the size of any of your partitions in the main hard drive at any time (even while using OS X). However, for some reason, Disk Utility doesn't let me do it. In fact, when I select my Time Machine partition, Disk Utility tells me that it can't be modified.

    Any explanation of why it doesn't work and how could I solve it?

    Thanks in advance!
  2. Fishrrman macrumors G5


    Feb 20, 2009
  3. iRock1 thread starter macrumors 6502a


    Apr 23, 2011
    Did you read?
  4. Weaselboy Moderator


    Staff Member

    Jan 23, 2005
    You cannot resize that bottom partition. What you need to do is resize (shrink) the top one to whatever you want, then remove the bottom one and add it back to fill the now larger space. This will of course wipe the data on the bottom, Time Machine partition.
  5. chown33 macrumors 604

    Aug 9, 2009
    Sailing beyond the sunset
    Here's a basic explanation of why it doesn't work the way you want.

    In general, a partition is represented as a starting point on the disk (a block number), and a block count. There's more to it than that, but these are the simplest and most fundamental elements.

    Once the fundamentals are established (starting block and block count), the partition is formatted as HFS+J, or FAT-32, or whatever filesystem is desired. A filesystem sets aside some blocks for the basic directory structure, while most blocks are set aside for holding content (data). The blocks set aside for data start out as free blocks, unused by any file. When you resize a partition, it's just increasing or decreasing the count of free blocks.

    Once the filesystem is laid down by formatting, it can't be moved. You can increase or decrease the partition's block count without needing to move the filesystem, because that just increases or decreases the filesystem's free-block count. As long as there's nothing in those blocks, you can increase or decrease their number.

    However, to move the partition's starting point, you'd have to copy all the raw blocks containing the entire filesystem to a new location. And that's what Disk Utility can't do.

    In short, you can't move the starting point of any partition, even when you can increase or decrease the partition's size.
  6. noisycats macrumors 6502a


    Jun 1, 2010
    The 'ham. Alabama.
    Thanks for that response. I finally understand what I haven't for years.

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