How do you partition a mac hard drive without losing data?

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If you are running a recent version of Mac OS X you can use the Disk Utility software found in the Utilities folder within the Applications folder. As with all disc based operations it is good practice to have a back-up copy of your content. From the Disk Utility Help page (The Help menu is your friend):


Creating new partitions on a disk

You may be able to create multiple partitions on your disk without losing any data. Each partition works like a separate disk. You may need to create a new partition if you want to install multiple operating systems on your computer. Partitions can also help you organize your files.
NOTICE:Back up your data before creating new partitions on your disk as a precaution.
To create new partitions on a disk:
1. Select the disk from the list at the left, and click Partition.
2. Select an existing partition in the Volume Scheme list, and click Add (+).
Disk Utility splits the partition into two, leaving the data in one of the new partitions. If the partition is less than half-full, Disk Utility creates two partitions of equal size. Otherwise, it creates one partition large enough for the data, and another partition with the remaining space.
3. Resize the partitions as needed.
You can drag the dividers between the partitions in the Volume Scheme list, or you can select a partition in the Volume Scheme list, and then enter a value in the Size field.
4. Choose how to format the partitions that will be erased or created.
For each new partition, select it, enter a name, and then choose a format.
5. Click Apply.


Or you can get under the bonnet and do all that geeky stuff:

You need to use the "diskutil" command line program, along with the "-resizeVolume" flag...
Here is the help information for this command:

#################

Disk Utility Tool
Usage: diskutil resizeVolume [Mount Point|Disk Identifier|Device Node] size <part1Format part1Name part1Size> <part2Format part2Name part2Size> ...

Non-destructively resize a disk. You may increase or decrease its size.
When decreasing size, you may optionally supply a list of new partitions to create.
Ownership of the affected disk is required.
Valid partition sizes are in the format of <number><size>.
Valid sizes are B(ytes), K(ilobytes), M(egabytes), G(igabytes), T(erabytes)
Example: 10G (10 gigabytes), 4.23T (4.23 terabytes), 5M (5 megabytes)
resizeVolume is only supported on GPT media with a Journaled HFS+ filesystem.
A size of "limits" will print the range of valid values for the current filesystem.

Example: diskutil resizeVolume disk1s3 10G JHFS+ HDX1 5G MS-DOS HDX2 5G

Valid filesystems: "Case-sensitive HFS+" "Journaled HFS+" "Case-sensitive Journaled HFS+" "HFS+" "HFS" "MS-DOS FAT32" "MS-DOS FAT16" "MS-DOS" "MS-DOS FAT12" "UFS" "Linux" "Swap"

#################

So here's what you need to do: (if you want to install a Linux partition)

In the example from the help command, they are creating a second and third partition on the disk - you don't need to do this (because diskutil doesn't create ext3 or ext4 partitions)...leave the space as "free" or unpartitioned space.

When I'm giving you commands below, a normal shell prompt is shown as a "$" (you don't need to type in the $)....

1.) MAKE SURE YOU AREN'T RUNNING ANY OTHER PROGRAMS WHEN YOU'RE RUNNING THESE COMMANDS. YOU WILL NEED TO RESTART THE MACHINE IMMEDIATELY AFTER THE PARTITION GETS RESIZED.

2.) Use the diskutil command line program to determine which device number your startup disk is:

$ diskutil list

This will list out information that shows device numbers (ie. disk0, disk1s5, etc.). Determine the identifier for your hard drive (it will be listed under "name" - you probably only have one partition, and it's probably the last on on the list).

/dev/disk0
#: type name size identifier
0: Apple_partition_scheme *149.1 GB disk0
1: Apple_partition_map 31.5 KB disk0s1
2: Apple_HFS Macintosh HD 148.9 GB disk0s3

3.) You need to determine how much free space you have available, and how much you want to split off for Linux (you will want to leave a little for the Mac side, but have enough for Linux or whatever you are doing)...For the most part, if you "Get Info" on the hard drive, and it shows that you have 95 GB capacity, and that 40 GB are used, 75 GB would be a good size to resize the Mac partition to (giving you 35 GB of space there, and 20GB free space for LInux)...

4.) To do the actual resizing, you'll need to use the "sudo" command to run the command as root:

$ sudo diskutil resizeVolume disk0s3 75GB
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