Hotplugd is crazysauce. So much you can do with it. Toad claims to need kern.usermount so it won't work with a default install, and will be toadally broken once this option is removed in OpenBSD 6.0. I can't speak for all OpenBSD users, but I just end up putting my sd-card reader (which sees most FAT formatted cards at sd1i) and the first available USB external drive (again, usually sd2i for FAT) in my /etc/fstab and call the mount with doas.

doas mount /sdcard

Not that big of a deal.