I have installed ubuntu 14.04 successfully on my Dell Latitude 3550 and I have been using it more or less without problems for more than a week. I had completely erased windows, therefore there are no windows partitions on my diskanymore. Due to a problem with the touchpad, I tried ubuntu 15.10 by booting from a usb stick. The system booted smoothly by choosing it from the F12 menu at startup. Everything was fine, including the touchpad.

When I tried to reboot, however, the laptop started system checks instead of booting. The test showed that no bootable medium was found. I tried to repeat the steps from this page  in order to (re)create a boot option from the BIOS but when restarting the BIOS still could not boot from the disk.

I had to reinstall the system from the beginning...now ubuntu 15.10 it is...

therefore the question is: When one boots from a USB why the system does not boot anymore? Is there anything obvious that I am missing? Should I never try again booting from an external USB disk?

(this happened during my first day with the laptop, when I tried to reinstall windows 8.1 using the DVD that came with it from an external DVD connected via USB, but I did not pay too much attention to it since I installed ubuntu 14.04 from the beginning)