After creating a Windows Boot Camp partition on a Mac, you might eventually run out of space and want to make it larger. Here’s how to do it without reinstalling Windows. ***Remember to back up first, because a mistake here could erase all data on your machine!***

Step 1. By default, newer versions of OS X have System Integrity Protection, an anti-malware feature that doesn’t let you perform certain modifications to your partitions. Reboot to your Recovery Partition, start the Terminal, and type:

csrutil disable


Step 2. In OS X, shrink the size of your Mac boot partition using the Disk Utility GUI, and reboot.

Step 3. OS X will automatically format the new unused space as HFS+, and we want this space available for Windows, so go to the Terminal and type:

diskutil list

Find the partition you want to erase—probably disk0s4, but double-check that the size is correct—and then type:

sudo diskutil eraseVolume Free\ Space null disk0s4


Step 4. Right now your Windows partition will be temporarily rendered unbootable. To fix it, while still in OS X, download and install the gdisk utility.

Step 5. Type:

sudo gdisk /dev/disk0


Step 6. ***If you get any error messages at this point, stop!*** Otherwise, carefully enter the following (adapted from this forum thread):

r <enter>    go to the recovery & transformation menu
h <enter>    create a new hybrid MBR
5 <enter>    add partion 5 to the MBR
y <enter>    set up EFI boot
<enter>      accept the default MBR hex code of 07
y <enter>    set the bootable flag
n <enter>    do not protects more partitions
o <enter>    review (but don't apply) the changes


Step 7. If this went according to plan, you should see two partitions: one type EE and one 07, with the 07 entry marked with * under Boot. If this is what you see, apply the changes by typing:

w <enter>    write partition table to disk


Step 8. Now reboot into Windows and install the free version of Partition Wizard.

Step 9. Click on your Windows partition, click Expand Partition, and choose the unallocated space you erased in Step 3.

Step 10. Click apply and then restart Windows to complete the changes. You should now see the new space added to your Windows partition.

Step 11. Optional final step, reenable OS X SIP. Boot to your Recovery Partition, start the Terminal, and type:

csrutil enable


Step 12. Troubleshooting: If something has gone wrong and Windows is no longer bootable, try these repair steps to recover.