I have an old MBP with 128 SSD. And the free space is only a few GB left so it is unable to create a virtual machine or dual boot with Boot Camp. Is it possible to run Windows 10 Mac from en external USB drive?
Yup,it's called windows to go, it's a feature of windows enterprise and education but you can find guides online to do that with any version of windows.
You will need a fast device though if you want it to be usable.
As a Windows To Go Creator Wizard alternative, WinToUSB (also called Windows To USB) allows you to install & run Windows on USB drive or Thunderbolt drive or clone existing Windows to USB drive or Thunderbolt drive, with it you can easily create a portable Windows 10/8/7 on an external hard...
If you make an MBR legacy bootable copy of Windows, the computers BIOS must be able to boot to an MBR legacy version to work
Make a separate UEFI copy of Windows for systems that only boot to UEFI and keep them labelled to avoid confusion
Make sure the hardware can boot to the specific boot format you chose when making it . Also if you want to run windows from USB on mac, there are many tools can do it like rufus, wintousb and unetbootin.