Remove GRUB from macOS Catalina
With Ubuntu, GRUB files are stored in the EFI/BOOT and EFI/ubuntu folders in the hidden EFI partition. Apply the following steps to remove the GRUB files.

Enter the following command to determine the identifier for the hidden EFI partition on the external drive.
diskutil list

For this example, I will assume the identifier is disk2s1. In the following steps, make the appropriate substitutions.

To mount the volume stored in the EFI partition, enter the following command.
sudo diskutil mount disk2s1

Note the volume name given in the output of this command. For this example, I will assume the volume name is EFI. In the following steps, make the appropriate substitutions.

Use the Finder application to view the folders in the EFI folder of the EFI volume (device). See image below.


Move the folders BOOT and ubuntu to the trash.

Empty the trash.

Enter the following command to unmount the EFI volume (device).
diskutil unmount disk2s1