Why my Mac Mini doesn't boot with external HDD attached?

Ext HDD is 3,5" 4TB WD RED (exFAT) in USB 2.0 box with external power supply (220v)

When I restart my Mac Mini (late 2012, MacOS 10.14) in 90% the boot will end on black screen (no apple loading logo, no progress bar). When I turn off or disconnect my ext HDD (exFAT), or reset PRAM then the system will boot perfectly. Then I can reconnect my extHDD and everything will work (till next restart).

Solution 1:

Assuming you have not installed a third party dual boot loader such as rEFInd, go into the System Preferences and select the internal HD as the "Startup Disk." If you have installed a third party boot loader it may be damaged or you may need to uninstall. If you don't know what a third party boot loader is; you probably don't have it.

Solution 2:

So problem was with extHDD on exFAT. After reformating to NTFS is everything OK.