Can't add external hard drive

You have to format the drive using Disk Utility (Applications - Utilities - Disk Utility) to APFS. If you don't know how please read below:

1. Open Disk Utility To open Disk Utility - Open Finder > Applications > Utilities > Disk Utility
2.  Click View in the upper left and select Show All Devices
3. Select the non-indented entry, you will likely see Toshiba in the name
4. Click the button labeled Erase in the menu which will bring up this window. NOTE: If you don’t see scheme, then make sure that all devices are showing and the non-indented entry is selected. Check step 2 to confirm Disk Utility is Showing All Devices
5. Provide a Name for the volume such as Time Machine
6. Set Scheme to GUID Partition Map
7. Set Format to APFS NOTE: Make sure that Scheme is set to GUID Partition Map and not Master Boot Record (MBR). If APFS is not available then the drive is in MBR and not GUID Partition Map. See previous step.
8. Click Erase, Disk Utility will start formatting the drive
9. Once complete, click Done
10. The drive should now be formatted APFS, this can be confirmed by selecting the indented entry which will now show APFS Volume under the name you provided

Hi,

Did you reformat the external drive with APFS? And if you try to use your external drive as TimeMachine backup, open System Preferences > TimeMachine, then sets your external drive as Time Machine backup drive.

If not, popup menu for TimeMachine drive, click on Not button.