What are 'Other Volumes in Container?'

Ever since the launch of High Sierra, macOS devices have been using the APFS file system. Its structure doesn't rely on disk partitions anymore and is instead built around containers. Users can have multiple containers on a single physical disk, and each container can further be split into several volumes. To illustrate this, your macOS startup container includes the main boot volume (Macintosh HD), Recovery volume, Preboot volume and more. 

The specific message 'other volumes in container' is displayed in the 'Storage' tab found under the 'About this Mac' setting.

1. Go to the Apple Menu.
2. Select 'About this Mac.'
3. Go to the 'Storage' tab.

After waiting for the Mac to finish calculating, users will see a bar with different colors showing the exact way that the storage on the device is allocated. Sometimes, there will be a gray box labeled 'other volumes in container.' These usually include the recovery, preboot, and any virtual volumes. These are core components of the macOS and cannot simply be deleted to free up space. Instead, try the solutions outlined below.

Create A New User Account

1. Go to the Apple menu.
2. Select 'System Preferences.'
3. If you need to, unlock the window by clicking on the padlock icon and entering the credentials for your admin account. 
4. Now, press the '+' (plus) symbol located at the bottom left of the screen.
5. Select 'Administrator' from the options in the dropdown menu.
6. Enter the required details for the new account.
7. After creating the account, restart the Mac and log in using it.
8. Go to the Apple menu again but this time select 'About this Mac.'
9. Select the 'Storage' tab and check if the 'other volumes in container' box has disappeared.

Delete Extra Volumes  

To find more details about the container and the volumes in it, users will need to open the Terminal tool. Once it is running, type the diskutil list command and execute it. You should be presented with a detailed breakdown. Look for any extra or duplicate volumes. 

If you find any, delete them by following the instructions:

1. Open Disk Utility.
2. Select the APFS volume you wish to delete.
3. Click on the 'Delete Volume' button in the toolbar. If the button is unavailable, it means that the selected volume is currently in use and can't be deleted. In this case, you will have to run Disk Utility in macOS Recovery.
4. To start macOS Recovery, shut down your Mac. Then while booting it back up, press Command+R on your keyboard. You can release the keys when an Apple logo appears on the screen. 
5. Select Disk Utility from the available options and delete the extra volume.