Problem
Boot Camp Assistant stalls when partitioning. Disk Utility is unable to format the drive.

Causes
Previously had partitioned the drive with Boot Camp Assistant, but ended up deleting the partition with Disk Utility. That was a mistake as Disk utility doesn’t seem to be able to handle CoreStoreage Logical Volume Groups that BCA creates to handle a 3TB drive.

Solution
List out the CoreStorage LVG:

diskutil cs list

Take the UUID of your primary drive and delete the LVG:

diskutiil cs delete [UUID]

Disk Utility was then able to format, and BCA was able to properly partition the drive.

Information Sources