External SSD has "Owners: Disabled" - can't Partition

Select that disk's Desktop icon and press COMMAND-I to open the Info window. At the bottom is the Sharing and Preferences panel. Click the little lock icon and authenticate. Uncheck the Owner's Disabled box at the bottom. Problem is solved. This isn't necessary if the disk will not be used for booting the computer.

For a new disk try:

Drive Partition and Format - El Capitan or Later

1. Open Disk Utility in the Utilities' folder.
2. After Disk Utility loads select the drive (out-dented entry with the mfgr.'s ID and size) from the side list.
3. Click on the Erase tab in the Disk Utility toolbar. A panel should drop down.
4. In the drop down panel set the partition scheme to GUID. Set the Format type to Mac OS Extended (Journaled.)
5. Click on the Apply button and click on the Done button when it is activated.
6. Quit Disk Utility.

If you want to make more than one partition, the skip Step 6. Instead, select the volume you created (indented entry in side list) then click on the Partition button in the toolbar. You will see a pie chart representation of the disk. Under the chart you will find a plus and minus button. Click on the Add [+] button to create a second partition. You can click the button as many times as you need partitions. Adjust each partition's size by clicking in the partition then changing the size using the adjusting handle in the pie or using the Size: field to the right.

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These are typical settings when Ignore Ownership is disabled:

New versions of macOS assign Read only to the group named wheel. The above, however, is more useful because the Admin user is part of the wheel group.

Make the above changes and you shouldn't have a problem partitioning the disks.