Missing boot loader (after installing TrueOS)

I have a 1TB USB disk on which I installed FreeBSD some time ago, but it only used up 50GB having left plenty of space for other OSes.
Now, I've installed TrueOS, thinking it would install the rEFInd boot manager, but on reboot I am greeted with a 'Missing boot loader' msg.

After booting from another disk I ran:- gpart bootcode -b /boot/pmbr -p /boot/gptboot -i 1 da0 thinking it would reinstall the original boot loader but it made no difference.

Can anyone suggest what I should do?
 
Careful, don't just blindly install gptboot(8). It's not used when EFI booting a system, that's what the efi partition is for. So, check what you actually have, gpart show. And verify what partition 1 actually is, I hope you didn't overwrite something important.
 
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