The error from Boot Camp means that it cannot find 50 GBs of contiguous space at the end of the drive in which to place the partition.
If it cannot, then the only solution is to defragment the drive. A simple solution if you have an external drive your can use:
1.
Repairing the Hard Drive and Permissions
Boot from your OS X 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 Installer menu (Utilities menu for Tiger, Leopard or Snow Leopard.) 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. Now restart normally.
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.
2.
Clone using Restore Option of Disk Utility
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.
3. *Boot from the external drive*: Restart the computer and after the chime press and hold down the OPTION button until the boot manager appears. Select the icon for the external drive then click on the downward pointing arrow button. The computer should boot from the external drive.
4.
Restore the Clone
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.
Source means the external backup drive.
Destination means the internal startup drive.
Shut down the computer, disconnect the external drive, then restart. The computer will start from the internal drive with your old system all intact and the hard will be defragmented. Boot Camp Assistant should be able to make the 50 GB partition you want.
