The problem was more worked around than resolved.

After having no grub modules with an installation on a new disk, whether
DVD or netinst ISO, I knew the problem wasn't with the disk. I installed
Devuan Jessie more or less successfully on a new disk without the old
disk being connected. Then I reconnected the old disk and now can boot
either operating system successfully.

If anyone has an idea of what might be the problem, I'd appreciate
knowing. Perhaps with both disks attached, the presence of one
preoccupies the SATA port needed by the other.

Incidentally, the key on which the ISO is installed names itself UEFI. I
have both UEFI and legacy selected in BIOS. If I have just legacy,
nothing boots.

Haines Brown