Hi
lotuska,
First of all, welcome to the iMore Forums.
Let's first check the partition scheme that your external drive is using.
The easiest way, but not the only way, to do so is:
- Click on the menu in the top left of your screen.
- While having the Option (⌥) key pressed, choose System Information…
Note: System Information… will not appear unless the Option key is pressed. - In the System Information window, select USB in the left sidebar.
This will list all the USB devices connected to your computer - including internal like the FaceTime camera if your computer has one.

If you use another kind of drive like Thunderbolt, select that instead 
- Go through the different devices until you see one that lists your drive's name. Then look at the line that says Partition Map Type, that's the partition scheme.
In my screenshot (highlighted), that is GPT (GUID Partition Table which is what Intel and I believe Apple Silicon Mac computers require to start - and what is used on all new Windows computers as well.

Partition schemes can be changed, but it does require erasing the entire drive, since they determine how the volumes are written on the drive itself.