i take it you did not heed all the warnings about installing to a usb
make 100% sure that the os installing on the usb puts the bootloader ON the usb
reinstall fedoras grub to the drive
"install-grub2 /dev/sdb1" but that doesn't seem to work.
is sdb1 your fedora boot partition ? ( first partition on the second drive )
if so what is on sda1 ?
is sdb your usb ?