Cannot resize partition...

Usually when I see "Cannot resize..." it is because the Windows partition was too small, or they need more room for OS X files. Not because they got the error message from Boot Camp Assistant (which tells you to reformat and restore after you have a backup).
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Step #1 is (always) backup before making changes.
Real easy... buy an external, you will need backups for Mac OS and probably Windows. you will want to be able to run OS X off external too at some point.

It isn't because there are big files, it is that they can't be moved.

The free space has to be contiguous, not fragmented, also, in order to create a slice for Windows.

I assume it won't work with a VM running under/inside OS X so you need to run Windows natively.

Some people have bought iDefrag ($49?) and results are anywhere from good to mixed to didn't work.

You are right, the question has been asked... about 999 times in the last two years.

Hi,

There are many options given on the net, many are time consuming and may not work (trust me i've tried).
An option that worked for me is to run a disk repair with Disk Utility.
I tried partitioning manually after boot camp failed and got the message - "error 0".
If i remember correctly, this is some sort of file mapping error in the HD which is fixed with Disk Utility, and now Boot Camp can make any partition size for me.
Hope this works for anyone else having this problem and saves the time of backing up and reinstalling.

If you can't afford to backup then you are really going to wish you had.

Can't help.