MacBookPro 2015 not booting from a copy of the disk of my old MacBookPro mid 2012
First perhaps boot in verbose mode to see what is causing it not to boot. If it is hardware incompatibility like missing driver (which can happen if the install has continuously been upgraded and so not managed to get the drivers for other Macs) then the following non-destructive steps (assuming you don't care about what is currently on the 2015's internal SSD) should fix it:


On the 2012 Mac download Sierra and make a bootable installer
on at least an 8GB USB drive.
Boot the 2015 using the installer USB
and use Disk Utility (from the menu bar) to clone the external SSD on to the internal
SSD (be very careful to get this direction correct).
Disconnect the external SSD to keep it safe.
Quit Disk Utility and run the Sierra installer over the top of the current un-bootable installation.


That should add in all the necessary drivers for the new Macs and it should work again. It's perfectly fine to reinstall macOS it won't delete any applications or data. Good luck!