MacOS – OSX Yosemite can’t resize partitions

Here is my current solution:

• boot from OS X DVD - mandatory!
• open terminal
• diskutil list
• umount "/Volumes/Macintosh HD"
• fsck -f /dev/disk0s2
• diskutil resizevolume /dev/disk0s2 100G

Lion has a hidden partition which contains recovery information. Shrinking the partition, according to an Apple employee, is not supported. He hooked up my MBP to an external hard drive and wiped and reinstalled Lion in order to fix things.

In more detail, what happens is this: When you try and resize the main partition, it doesn't include the recovery partition. When you hit "Apply Changes," it does a checksum to make sure that nothing's accessing the drive. It then gets started, doing another checksum, and discovers that something's changed, breaking the previous checksum.