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In my disk utilty, I have some unused space left over.

I want to resize Macintosh HD to use this space.

Whats the best way to resize Macintosh HD? Disk Utility won't let me resize it whether I'm live resizing from the tool running on my Mountain Lion install or if I boot into Recovery HD so the OS isn't running.

Here is a screenshot of my Disk Utility window currently

Disk Utility showing volume

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So you have a partition that is smaller than the size of the disk, and you have some unformatted/unpartitioned space left? How much space? –  paulgrav Dec 26 '12 at 22:04
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4 Answers 4

Please please make sure that you type in the correct partitions. Double-check and triple check before running the merge command. It’s really important that you backup all of your data. I’m not taking any responsibility if you manage to erase anything. You will not want to lose data if there’s a sudden loss of power mid-operation.

  1. In Disk Utility, create a new partition from the unused space.
  2. Open terminal.app, and from there you’ll run diskutil mergePartitions

First you need to find out which partitions you’ll want to merge:

disktuil list

And hopefully you’ll see something like this:

/dev/disk0
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *500.1 GB   disk0
   1:                        EFI                         209.7 MB   disk0s1
   2:                  Apple_HFS Macintosh HD            499.2 GB   disk0s2
   3:                  Apple_HFS Untitled                 50.0 MB   disk0s3   
   4:                 Apple_Boot Recovery HD             650.0 MB   disk0s4

You’ll want to merge disk0s2 with disk0s3. So you’d run something like this:

diskutil mergePartitions JHFS+ DiskName disk0s2 disk0s3

Beware that the above will merge partitions between disk0s2 and disk0s3. Data on disk0s2 is preserved, data on all the partitions in-between is erased. In my example the two partitions are contiguous so there’s no problem. But if your recovery partition is in between it will be erased.

If you need help with the mergePartitions command:

Usage: diskutil mergePartitions [force] format name DiskIdentifier|DeviceNode DiskIdentifier|DeviceNode

Merge two or more pre-existing partitions into one. The first disk parameter is the starting partition; the second disk parameter is the ending partition; this given range of two or more partitions will be merged into one.

All partitions in the range, except for the first one, must be unmountable.

All data on merged partitions other than the first will be lost; data on the first partition will be lost as well if the "force" argument is given.

If "force" is not given, and the first partition has a resizable file system (e.g. JHFS+), it will be grown in a data-preserving manner, even if a different file system is specified (in fact, your file system and volume name parameters are both ignored in this case). If "force" is not given, and the first partition is not resizable, you will be prompted if you want to erase.

If "force" is given, the first partition is always formatted. You should do this if you wish to reformat to a new file system type.

Merged partitions are required to be ordered sequentially on disk. See diskutil list for the actual on-disk ordering; BSD slice identifiers may in certain circumstances not always be in numerical order but the top-to-bottom order given by diskutil list is always the on-disk order.

Ownership of the affected disk is required.

Example: diskutil mergePartitions JHFS+ NewName disk3s4 disk3s7 This example will merge all partitions BETWEEN disk3s4 and disk3s7, preserving data on disk3s4 but destroying data on disk3s5, disk3s6, disk3s7 and any invisible free space partitions between those disks; disk3s4 will be grown to cover the full space if possible.

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'If "force" is not given, and the first partition is not resizable, you will be prompted if you want to erase.' Considering that DiskUtility does not allow re-size, sounds like it will not be allowed. –  arin Dec 28 '12 at 16:19
    
I think Disk Utility doesn’t allow resizing because Apple doesn’t want end users resizing partitions. The condition is not whether the “partition” is resizable, but rather the file system — it needs to be JHFS+. Fortunately that’s the default. –  paulgrav Dec 28 '12 at 16:26
    
Might be, I am very interested in whether this would work. Hopefully the OP will keep us updated :) –  arin Dec 28 '12 at 16:29
    
I'll try this method tomorrow. I'll do a Time Machine backup beforehand and give your tip a go. I'll post back tomorrow –  Jamesking56 Dec 29 '12 at 21:55
    
Did it work? Does someone know how to reorder the partitions because with me *disk0s2 is my Macintosh - main partition *disk0s3 is my Recovery HD *disk0s4 is my 2nd partition that i want to merge with main partition –  tdhulster Feb 27 '13 at 23:45
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This is typically the result of the Macintosh HD being a core storage partition and not a standard partition that sits directly in the partition table.

If you have turned on FileVault - this will convert your Mac to use Core Storage. You can also type the command in terminal to see whether my guess is correct: diskutil cs list

If you don't have Core Storage set up and have this inability to resize a volume, then something is amiss with the volume structure, as you should see a window like this: Disk Utility with resizable partition

If this doesn't do the trick comment or edit in what diskutil list shows on the problematic Mac and there could be an easy command line solution to resizing things as well.

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I don't have FileVault switched on. I can drag it down from the corner but when I hit apply it doesn't apply the changes. It gives an error. –  Jamesking56 Dec 29 '12 at 21:56
    
Under window -> show log are there any errors you see or want to incorporate in your question? Also, make sure the Partition Map Scheme is GUID Partition Table. –  bmike Dec 29 '12 at 23:37
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up vote 1 down vote accepted

OK Everyone! I've finally got around to fixing my Mac! For future reference, here is how I did it!

Firstly, I redownloaded the Mountain Lion installer from the App Store (as it is a digital download now) by holding Option and clicking 'Purchases' to make it appear (after reading a forum online).

After doing this, I created a new blank partition on my spare drive ready to create my own bootable Mac OS Mountain Lion installer.

Next, I downloaded a free app called 'Lion DiskMaker 2' (http://liondiskmaker.com) which took my installer and create a bootable drive out of it using my blank partition.

After that I created a Time Machine backup.

Then I booted into my new Mountain Lion installer and completely reformatted my Macintosh HD back into the 1 partition format.

Then I installed Mountain Lion onto that using my Time Machine backup.

And here I am back with the 1 partition and now using the entire drive! Lengthy process but got there in the end!

So if anyone else has got this problem, this is how to fix it! :)

Thanks for all of your help anyway!

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You can't resize an existing partition in Disk Utility. Your only options are

  1. Create a separate partition to make use of the unused space.
  2. Copy all your data to a backup drive, then erase and repartition the disk, then move your data back (when reformatting, be sure to select the drive, not just the existing named partition, or you will end up with the same unused disk space).
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OS X has had live resizing of HFS volumes for some time. Perhaps you are referring to non-native file systems like FAT? –  bmike Dec 28 '12 at 6:15
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