Start notebook from external hard drive?

I cloned my entire (old) hard drive and installed it in an external housing. How can I boot the notebook from the external hard drive?

The identical operating system is found on both disks (Win 10)

Your question indicates that you are not very familiar with the Windows boot procedure. The way you imagine it doesn't work!

To boot, the PC not only needs the operating system, but also the boot partition (a hidden partition on the internal hard drive). All systems that are ever to be started on the PC must be "registered" in this boot partition.

In order to be able to boot an operating system from an external hard disk, this system must first be "registered" in the boot partition of the internal hard disk. It works like this:

Press the Windows key + R to open the Run mask.
Enter the command diskpart here and confirm with OK.

The command prompt (cmd.exe) opens with the DISKPART> function. Now type in the command list disk and confirm with Enter.

All data carriers are displayed and a number is assigned to them. Now type in select disk NUMBER (the number assigned to the desired hard disk) and confirm with Enter.

If the desired hard disk has several partitions, also enter the command list partition and confirm with Enter.
The partitions are displayed. Make a note of the number assigned to the system partition. Then type in select partition NUMBER (the number assigned to the system partition) and confirm with Enter.

Now type in active and confirm with Enter.
Close the window by entering the exit command and confirm with Enter.

When all windows are closed again, perform a restart.

During the restart that follows, you call up the boot manager using the appropriate key combination (usually F11 or F12) and select the external hard drive that is now displayed. If everything has been installed correctly, your system should start from the external drive!

Note 1: "Booting from a USB medium" may first have to be enabled in the BIOS / UEFI.

Note 2: If you have copied your hard drive 1: 1, then the "old boot partition" is now also on the external hard drive. This can't be seen because it is hidden. If the Windows system was installed in its own partition (see point 5 above), then there are no problems. The boot manager only accesses the system partition.

However, if the entire hard disk consists of only one partition, problems can arise because the entire hard disk was then "registered" as a system partition. The boot manager then suddenly finds two boot partitions and it can't cope with that! In this case the "old boot partition" on the external hard drive must be deleted (Control Panel \ Administrative Tools \ Computer Management \ Disk Management). The boot partition is approx. 500MB in size and is called (by default) "system reserved". But CAUTION! Under no circumstances should you delete the boot partition of the internal hard drive! - Then nothing works anymore!