The HC4 odroid images (official focal and meveric's armbian-buster (and I think toogood's images too) don't have zfs in the kernel (I think because zfs isn't in the mainline kernel either?)
Installing zfs-dkms fails because there's some incompatibility between zfs-dkms 0.7.12 in focal and buster.
I had a play with git clone and building openzfs master. That correctly built the kernel module and importing the pool and running zpool and zfs-test all succeeded.
Meveric, however, has pointed me at:
apt install -t buster-backports zfs-dkmsBecause
"I checked however on the ZFS DKMS module form Debian Buster and found
the version that is in the default repository (version 0.7.12 of
spl-dkms and zfs-dkms) are not compatible with Kernel 4.9 that we're
using.
Luckily for you in buster-backports there's version 0.8.5 available and THAT is compatible with Kernel 4.9."
Many thanks to Meveric for the assist. While I could have continued with what I'd built from source, it's better to use the backported spl-dkms so that I can get future updates/fixes.
https://forum.odroid.com/viewtopic.php?p=313882#p313882
