Can I boot an external SSD in the safe mode

[Mac Hammer Fan]

If an external SSD is connected to an iMac 2015 using the USB 3.0 port, can I boot in the safe mode (shift)?
Or will the Mac Boot from the slow internal drive? (Just in case the Mac has startup problems when booting from the SSD)

 

[mikehalloran]

If the external is selected as the boot drive, Safe Mode will work.