Mac Crashes During Boot Camp Assistant Disk Partitioning
209 GB usable space is normal for a 256HD.  It has to do with formatted partition sizes plus the hidden recovery partition.  Totally normal.
It will likely crap out again.  Not sure why.  To fix it, you'd need to look at log files (Applications/Utilities/Console) system.log and see if there's any error or at least what its doing.  Fixing it will be a total PITA.  The most direct way to fix it, will be to back everything up, boot from an Internet Recovery (not the local recovery) and totally repartition the drive using Disk Util.  Then put the Mac OS back and your data back.  Then do it again.  
Hold Command-R at boot to boot up the hidden recovery partition.  In there, use Disk Utility to check the disk for errors.  Don't format, partition, or erase your disk there - that'll kill all your data.  Hopefully, you have a full backup before starting any of this process.  good luck!