Why does my Mac not see my external drive as a boot option unless connected via an Apple-branded dongle
Why does my Mac not see my external drive as a boot option unless connected via an Apple-branded dongle?


Low quality USB bridge.

Inside the dongle, there's an IC (integrated circuit) that bridges the USB bus on your computer with the peripheral device.  Off brand manufacturers look for every edge to increase their profit margin so they use components that aren't exactly up to par. 

Personally, I'm very wary of plugging in an off brand, low cost peripheral device into what is the highest priced laptops/desktops on the market today.  

There was a time when USB cables were being made so poorly they could actually kill your device.  This is why you go with reputable brands.  I'm partial to Anker (they make an equivalent to the Multi-Port AV Adapter) because they offer an 18 to 24 month warranty on all products.