From your previous posts I understood that your laptop is an old one, so it is not a given that is able to boot from USB. Even if it is able to, the BIOS in your laptop could be configured in such a way that only a subset of USB devices (keyboard, mouse, pen drive) is allowed to boot from. Computer manufacturers do this to make their computers boot faster. Most modern computers allow you to change this behavior, but not all of them.
Is your laptop able to boot from a portable hard disk on any USB port? That would be my first question if you insist on booting from a portable hard disk. Even if you manage to boot from an external hard disk...it makes use of the USB port, which is in each and every case much, much slower than the internal SATA port which the internal hard disk uses.
In other words, if you think your laptop is slow with the internal hard disk, with USB based external devices it will become like the slow nephew of a snail. Or, if you need a visual: USB would be a the size of a drinking straw, SATA would be the size of a garden hose, PCI Express would have the size of the sewage pipe that connects your house to the main sewage system. Imagine data to be water and guess which will be faster moving water in the shortest amount of time.
External devices are ideal for simply creating a backup rather quickly...but using it as a boot device? Only for (re-)installing the operating system, nothing more.
A backup in my book is a copy of a file that is not stored on the computer it normally resides. Nothing more, nothing less.
What you can do still is turn the laptop completely off, removing its hard disk, open the enclosure of the external hard disk, remove the hard disk from the enclosure and connect it to the laptop, close the laptop. If the clone action went well, the laptop should boot normally.