Installed USB drive booth problem

Welcome to the Ubuntu Forums

I think that you installed the bootloader into the inteal drive.

This works in BIOS mode:

The best method to avoid that is to remove the inteal drive, and then install Ubuntu into the USB drive. The second best method (more risky, but also more convenient) is to select Something else at the partitioning window of the installer.

Then you have to [create and] select the partitions manually, and at the bottom of the window, you will be able to select where to install the bootloader. Write it to the head of the USB drive, /dev/sdx, for example /dev/sdb. Do not use any digit (not /dev/sdb1) - which means that you write it to a partition, and that is not what you want.

If your computer is ruing in UEFI mode, things are more complicated. You can try according to the following method, iIf you want a pendrive with a live and an installed system, that works in UEFI and BIOS mode,

A new and so far successful attempt to create a stable portable system, that works in UEFI and BIOS mode

-0-

It is also possible to repair or re-install the grub boot-loader for linux according to the following link

Grub2/Installing

and there are corresponding tools to re-install the Windows bootloader.