Out of curiosity, is your host system OS X 10.11 (El Capitan) as well? From my experience, El Capitan's Disk Utility has been basically useless, and I've had several times like you describe where it just flat out refuses to let you do things. IF this is the case, I would recommend getting the old, less useless Yosemite Disk Utility back following the instructions here:
Option 1 - From a Backup
Disk Utility.app from the
/Applications/Utilities folder of a Yosemite Time Machine backup to somewhere convenient (like your desktop)
2. Download a hex editor (I personally like Hex Fiend)
3. Right click your copied Yosemite Disk Utility and click "Show Package Contents."
4. Go to
Contents/MacOS within the package contents and open the file called "Disk Utility" with your hex editor
5. Find the chunk
D584C00F 85440100 within the file (near or on line 25056 in Hex Fiend) and change the '85' in the second half to '84'.
6. Save the file. You should now be able to run this copied Disk Utility.
Note: you may have to give root:wheel ownership:
sudo chown -R 0:0 ~/Desktop/Disk\ Utility.app
Option 2 - From a Download
1. Copy your El Capitan Disk Utility somewhere else besides
2. Run this script, which downloads the Yosemite Recovery Update, copies out Disk Utility, and patches the version for use on El Capitan.
Note: It also tries to copy it over the current El Capitan Disk Utility, which failed on my computer because of system permissions.
IF your host is not El Capitan, the above does not apply. But as long as your host is a Mac, you could also try looking at the partition via the command line tool
diskutil, although in my experience in El Capitan this tends to work almost as poorly as the new Disk Utility GUI.
1. List your disk partitions with:
2. Find your VirtualBox partition on this list - it will probably be something like
sudo diskutil resizeVolume ` to try and resize the partition. If there is data on the partition, make sure you have it saved elsewhere.