If the external drive doesn't show bootable, verify that the installation of Ubuntu on it is UEFI-compliant -- it needs to be, since your system does not support legacy boot.  
Hi everyone, I just bought the new Dell Inspiron 3511 this week and I am a programmer. So basically I need to separate my work on a different SSD with Ubuntu 20. I turned on the notebook and it doesn't recognize when I press F12 or F2. I have already disabled secure boot in the bios. But when I start with my original SSD with Windows, I can see the Ubuntu SSD in the manager disks. What should I do?I need to plug in my SSD, press F12 and load Ubuntu to do my work, I use it on my CPU and it works great. Thank you very much. [IMAGE]   [IMAGE]   [IMAGE]   [IMAGE]   [IMAGE]  
External SSD is not detected on boot