My laptop hard drive died, so as an interim solution, I wanted to install Void Linux on a removable hard drive [/dev/sdb1]. 

I started with the current 64-bit MUSL live image.  The void-installer script, run as root, fails to complete, complaining about expecting to find an efi setup.  Rather than get involved with efi , I tried

  mkdir /usb
  mount /dev/sdb1 /usb
  <create /usb/boot/grub/grub.cfg with suitable menu item>
  grub-install --target=i384-pc --boot-directory=/usb/boot  /dev/sdb
  umount /usb

which completed without errors.  However, on reboot, I get the grub menu from the failed hard drive.

I also ran fdisk on /dev/sdb to mark the partition bootable.  What have I missed?  Thanks.