I have read in the past that Windows won't boot from an external drive.
Recently I changed laptops and I now have the old hard drive (V-NAND SATA) in a usb enclosure. It turns out that the new laptop will boot from the old drive (Win 10). I don't know whether it's booting via boot manager or via legacy boot.
I used Macrium Reflect to clone the internal drive of the new laptop to a new hard drive (NVMe M.2 PCIe) in another usb enclosure. The clone does not boot (this one is / would be Win 11). If I specify that drive as the first boot device the system does nothing for a bit and then boots from the internal drive. I cloned all the partitions except for an empty data drive (the external drive doesn't have room to clone the lot).
I read on one of the cloning software forums that it's a good idea to disconnect the internal drive if you are trying to boot from a clone, because the "drive signature" will also have been cloned and Windows can get confused. It's not practical to do that because the new laptop is quite flimsy and notorious for breaking if you take it apart. Is the problem likely to be due to the drive signature (not sure what that is) and if so how do I change it?
Otherwise, could the difference be down to the usb enclosure i.e. the type of usb device that is recognized by windows? Does it see one as removable storage and the other as just storage maybe? How would I check that? I would try copying the partitions of the internal drive to the old hard drive (the one I can boot from) but there isn't room.
