Boot from external hard drive is not displayed?

I would like to boot from an external hard drive. Which format of the file must be on the hard disk? I wanted to try it. I didn't see the hard drive on a laptop. On the other laptop under Temporary Start Device displayed at boot but nothing happens when I press Enter.

You have to 'burn' the file to the external hard drive so that you can start from it and you have to mark the hard drive as bootable, it is best to use BalenaEtcher, which burns and marks at the same time. The file must be in .iso format for burning.

After I selected the iso file, "It looks like this is not a bootable image. The image does not appear to contain a partition table, and might not be recognized or bootable by your device." What should. I make?

Hmm… Seems like the ISO file you want to burn is not bootable. You can press OK and still burn, but you may not be able to start from the external hard drive

What do you want to boot from an external drive plate? If you want to install Windows 10 on an external hard drive, you can proceed as follows: https://www.easeus.de/partitionieren-tipps/windows-10-auf-externer-festplatte-installieren.html

I actually want to install Mac on an old computer. How does it work?

Which iso file should i use?

For me, all internal breakfast plates are shown in the BIOS but no external ones.

Should I try a USB stick?

I read in the comments that you want to set up a Hackintosh. This is not possible without special software changes and also a breach of the terms and conditions.

If you really want to enforce it, you have to burn the system ISO to a USB stick, e.g. With Rufus. If the PC is too old (older 2004), it will make little sense.

Installing MacOS on a PC is actually not permitted and is also difficult to implement, see here:

https://tipps.computerbild.de/software/apple/macos-auf-pc-installieren-geht-das-ueberhaupt-589845.html

And how do I do it for a virtual machine such as: Hyper-V? For me it failed because of the graphics card.