Startup Manager doesn't consistently show all bootable disks

I have a couple of bootable disks on my MacBook Air, one the internal SSD and also an external USB disk. The USB disk is connected through a USB 3 hub. When I start the system, hold down Option, and get to the Startup Manager, I don't always see the USB disk, but see it about every other time. So, I have to hold down the power button to do a hard shutdown in order to start up again. I don't feel confident that I could install some other boot manager without screwing up the system and I like to keep it simple, so I'd like to use the Startup Manager.

Thanks.