Bootcamp can't partition my hard drive, what do I do?

You will have to backup your OS X partition to an external drive, boot from the external drive, use Disk Utility to repartition and reformat your hard drive back to a single volume, then restore your backup to the internal hard drive.

1. Get an empty external hard drive and clone your internal drive to the external one.
2. Boot from the external hard drive.
3. Erase the internal hard drive.
4. Restore the external clone to the internal hard drive.

Clone the internal drive to the external drive

1. Open Disk Utility from the Utilities folder.
2. Select the destination volume from the left side list.
3. Click on the Restore tab in the DU main window.
4. Check the box labeled Erase destination.
5. Select the destination volume from the left side list and drag it to the Destination entry field.
6. Select the source volume from the left side list and drag it to the Source entry field.
7. Double-check you got it right, then click on the Restore button.

Destination means the external backup drive. Source means the internal startup drive.

Restart the computer and after the chime press and hold down the OPTION key until the boot manager appears. Select the icon for the external drive and click on the downward pointing arrow button.

After startup do the following:

Erase internal hard drive

1. Open Disk Utility in your Utilities folder.
2. After DU loads select your internal hard drive (this is the entry with the mfgr.'s ID and size) from the left side list. Note the SMART status of the drive in DU's status area. If it does not say "Verified" then the drive is failing or has failed and will need replacing. SMART info will not be reported on external drives. Otherwise, click on the Partition tab in the DU main window.
3. Under the Volume Scheme heading set the number of partitions from the drop down menu to one. Set the format type to Mac OS Extended (Journaled.) Click on the Options button, set the partition scheme to GUID then click on the OK button. Click on the Partition button and wait until the process has completed. Do not quit Disk Utility.

Restore the clone to the internal hard drive

1. Select the destination volume from the left side list.
2. Click on the Restore tab in the DU main window.
3. Check the box labeled Erase destination.
4. Select the destination volume from the left side list and drag it to the Destination entry field.
5. Select the source volume from the left side list and drag it to the Source entry field.
6. Double-check you got it right, then click on the Restore button.

Destination means the internal hard drive. Source means the external startup drive.

Note that the Source and Destination drives are swapped for this last procedure.

Yes. Defragmenting the drive in this way will allow Boot Camp Assistant to find the necessary contiguous space to allocate to the Windows partition.

Try this:

Repair the Hard Drive and Permissions

Boot from your Snow Leopard Installer disc. After the installer loads select your language and click on the Continue button. When the menu bar appears select Disk Utility from the Utilities menu. After DU loads select your hard drive entry (mfgr.'s ID and drive size) from the the left side list. In the DU status area you will see an entry for the S.M.A.R.T. status of the hard drive. If it does not say "Verified" then the hard drive is failing or failed. (SMART status is not reported on external Firewire or USB drives.) If the drive is "Verified" then select your OS X volume from the list on the left (sub-entry below the drive entry,) click on the First Aid tab, then click on the Repair Disk button. If DU reports any errors that have been fixed, then re-run Repair Disk until no errors are reported. If no errors are reported click on the Repair Permissions button. Wait until the operation completes, then quit DU and return to the installer.

If DU reports errors it cannot fix, then you will need Disk Warrior and/or Tech Tool Pro to repair the drive. If you don't have either of them or if neither of them can fix the drive, then you will need to reformat the drive and reinstall OS X.

If you are using Lion or Mountain Lion then do this instead of the above:

Repair the Hard Drive and Permissions - Lion

Boot to the Recovery HD:

Restart the computer and after the chime press and hold down the COMMAND and R keys until the menu screen appears. Alternatively, restart the computer and after the chime press and hold down the OPTION key until the boot manager screen appears. Select the Recovery HD and click on the downward pointing arrow button.

Repair

When the recovery menu appears select Disk Utility. After DU loads select your hard drive entry (mfgr.'s ID and drive size) from the the left side list. In the DU status area you will see an entry for the S.M.A.R.T. status of the hard drive. If it does not say "Verified" then the hard drive is failing or failed. (SMART status is not reported on external Firewire or USB drives.) If the drive is "Verified" then select your OS X volume from the list on the left (sub-entry below the drive entry,) click on the First Aid tab, then click on the Repair Disk button. If DU reports any errors that have been fixed, then re-run Repair Disk until no errors are reported. If no errors are reported then click on the Repair Permissions button. When the process is completed, then quit DU and return to the main menu. Select Restart from the Apple menu.

After booting the Windows installer you will need to use the Windows formatter to reformat the Windows partition using NTFS. This cannot be done with OS X which cannot write NTFS.

If you print out the Boot Camp documentation it should explain everything for you. Open Boot Camp Assistant in OS X then click on the large Print Docs button.

I have struggled with every step in the process of installing Windows 7 via Bootcamp on my mid-2012 MBP running Mavericks. To finally get the partitioning process to work, I ran Disk Utility and repaired the disk, after which Bootcamp Assistant successfully partitioned the HD and installed Windows from the USB installer I had already created separately.

I also hit a snag in the Windows 7 installer where it said that the BOOTCAMP partition wasn't formatted for Windows. I clicked the "advanced" link and chose "Format" after which everything went smoothly.

Bootcamp can't create partition on iMac 21" Late 2013 with El Capitan 10.11.1

Resolved by

1. Command + S while booting

2. type command /sbin/fsck -fy

3. reboot

4. try using bootcamp again found no issue

all credit please give to https://www.youtube.com/watch?v=WmdsLfLAO20