Boot over external hard drive?

I removed my HDD from my laptop and installed an SSD in order to be able to use the PC again with reasonable speed. Now I have bought a case (with SATA) for the old HDD that can be connected via USB. Windows 8.1 was on the HDD, and I installed Windows 10 on my SSD. Can I now boot from the hard drive in the external housing and am I on the normal Win8 screen as before installing the SSD?

In addition, pictures, documents, etc. Are missing because they have been saved to a user and when I want to open this, a message appears with "No permissions".

So actually it should be possible without any problems since the device itself has not changed. Only have to let the external boot in the BIOS and then everything should be the same as before.

So, if you can't get into a boot menu or get into the BIOS when booting, then press Shift (Shift) and restart as usual (in Windows), then you can release the key and go to Troubleshooting in the BIOS / Uefi go to change your boot order in the boot tab.

I thought to myself, but it loaded forever. So I restarted the laptop and chose the SSD as the boot device again.

It is possible that this will be slower than before through the other interface, but that it doesn't work is actually strange.

I try again

Now there's an error message

May be because the drive mapping changes. C: is always the main loop, others and external ones are assigned to D: E: F:. Windows needs c: or possibly install a boot manager? (this was true for partitions). Should also give portable Windows versions. I have not tried them. I have portable Debian.

There are programs that are supposed to install Win10 on USB, but I had no success straight away, and I stopped the project

*the project.