You don't need a separate /boot partition/. That's an artifact of ancient history and no longer necessary or desirable. If you have successfully completed the installation, then simply rebooting the PC (by powering off then on again) is enough to bring it up in the new operating system.
If that does not happen, then I would suspect that the problem relates to the external USB-connected disk drive. It is not yet common for PCs to support booting from USB devices. If you know for a fact that your PC does have this capability, then you need to go into the boot configuration utility, and set the choice that tells the system to boot from USB disk instead of IDE disk.
Different manufacturers have different ways of getting into the boot configuration utility. It is typically done by pressing a function key early in the boot sequence. You probably do not need to make any configuration changes to GRUB. When you did the installation, GRUB was installed on the USB disk as part of the Linux installation.
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