Can't resize partition
WELL, HOLY CRAP.

I did some digging around in the manual pages for the diskutil command-line utility.  My volume was a CoreStorage Logical Volume, so I got some info on it:

$ diskutil cs list
CoreStorage logical volume groups (1 found)
|
+-- Logical Volume Group C8C10399-E3E0-4B52-A9EE-2F15AE56E992
    =========================================================
    Name:         Macintosh HD
    Status:       Online
    Size:         155701956608 B (155.7 GB)
    Free Space:   18960384 B (19.0 MB)
    |
    +-&lt; Physical Volume 297002AD-AB9E-47CC-B31B-5B7C21BE0F7A
    |   ----------------------------------------------------
    |   Index:    0
    |   Disk:     disk0s2
    |   Status:   Online
    |   Size:     155701956608 B (155.7 GB)
    |
    +-&gt; Logical Volume Family F134B1FE-CEC6-49F3-86E9-DF67D8530FF5
        ----------------------------------------------------------
        Encryption Type:         None
        |
        +-&gt; Logical Volume F85FD6EB-0CF7-400E-960B-22572D96B498
            ---------------------------------------------------
            Disk:                  disk1
            Status:                Online
            Size (Total):          155330674688 B (155.3 GB)
            Revertible:            Yes (no decryption required)
            LV Name:               Macintosh HD
            Volume Name:           Macintosh HD
            Content Hint:          Apple_HFS


Then I held my breath and ran a resizeStack command, which makes everything very easy:

$ diskutil cs resizeStack F85FD6EB-0CF7-400E-960B-22572D96B498 0
The Core Storage Logical Volume UUID is F85FD6EB-0CF7-400E-960B-22572D96B498
Started CoreStorage operation
Checking prerequisites for resizing Logical-Physical volume stack
Growing Logical-Physical volume stack
Verifying file system
Volume could not be unmounted
Using live mode
Performing fsck_hfs -fn -l -x /dev/rdisk1
Performing live verification
Checking Journaled HFS Plus volume
Checking extents overflow file
Checking catalog file
Checking multi-linked files
Checking catalog hierarchy
Checking extended attributes file
Checking volume bitmap
Checking volume information
The volume Macintosh HD appears to be OK
File system check exit code is 0
Restoring the original state found as mounted
Growing Core Storage Physical Volume from 155,701,956,608 to 499,248,103,424 bytes
Copying booter
Growing disk partition
Modifying partition map
Growing Core Storage data structures
Resizing Core Storage Physical Volume structures
Resized Core Storage Physical Volume to 499,248,103,424 bytes
Growing Logical Volume
Resizing Core Storage Logical Volume structures
Resized Core Storage Logical Volume to 498,876,821,504 bytes
Growing file system
Finished CoreStorage operation


Didn't even need sudo!  Which is kind of strange!

Anyway, after this, my disk is now in fact a 500 GB disk:



This is like those dreams where you discover extra rooms in your house that you didn't know you had.