I had this problem today. It turns out there was an a small issue with the directories on the disk which was easily resolved. To see if you have the same problem as me, try the following.
- Perform a full backup, just in case
- Launch "Disk Utility" (Applications > Utilities > Disk Utility)
- In the left-hand panel, click on your hard drive
- Click "Verify Disk"
- If there are any errors reported, then you may have the same problem as me (I had a directory count which was wrong).
- If there are any errors, try clicking "Repair Disk".
- If repair worked successfully, try re-running Boot Camp Assistant
If for any reason the repair option didn't work, here's another way to repair the disk .This is the technique which I used. It's a bit more 'technical', but quite easy. You can see the original artical on OSX Daily:
- Reboot the Mac into Single User Mode by holding Command+S during boot. You'll see lots of black and white text. Don't panic.
- At the command prompt, type: "/sbin/fsck -fy" (without the quotes)
- Wait while the disk is checked and repaired
- When fsck is complete, type “exit” or “reboot” to reboot
- Boot the Mac as usual, verify the disk again in Disk Utility.
- Run Boot Camp.
Hopefully this helps others, and will prevent costly visits to your local computer shop!