An alternative to the disabling encryption is to use command diskutil cs resizeStack
The details on how it works has been documented by Klanomath here
One thing I wanted to add is that it also works with FileVault. I have tried and the performance was pretty good. It took me 5-8 minutes to resize the 230GB partition to 250GB.
To sum it up:
Firstly: Boot from a separate system such as USB or the internet
Secondly: use this command diskutil cs list
to get the volume Id of the Logical Volume partition
I.e:
$ diskutil cs list
CoreStorage logical volume groups (1 found)
|
+-- Logical Volume Group A061B44A-9E3D-41B1-8943-C11BD76DDE37
=========================================================
Name: Macintosh HD
Status: Online
Size: 250337738752 B (250.3 GB)
Free Space: 180224 B (180.2 KB)
|
+-< Physical Volume 01F57DD6-2209-4FA6-A085-93C36C36B3D4
| ----------------------------------------------------
| Index: 0
| Disk: disk0s2
| Status: Online
| Size: 250337738752 B (250.3 GB)
|
+-> Logical Volume Family 777A289D-71C0-43A2-9C10-39EFEE313964
----------------------------------------------------------
Encryption Status: Unlocked
Encryption Type: AES-XTS
Conversion Status: Complete
Conversion Direction: -none-
Has Encrypted Extents: Yes
Fully Secure: Yes
Passphrase Required: Yes
|
+-> Logical Volume 11F03152-0861-4BA0-AA78-1F1D8CE520AF
---------------------------------------------------
Disk: disk1
Status: Online
Size (Total): 249999917056 B (250.0 GB)
Conversion Progress: -none-
Revertible: Yes (unlock and decryption required)
LV Name: Macintosh HD
Volume Name: Macintosh HD
Content Hint: Apple_HFS
It is 11F03152-0861-4BA0-AA78-1F1D8CE520AF
And you can use this command to resize:
diskutil cs resizeStack 11F03152-0861-4BA0-AA78-1F1D8CE520AF 250g