Comment 2 for bug 1816240

The boot loader is stored on sda since 2014 and used by all systems on sdc and sdd. Up to now all 18.04.2 systems use Linux 4.15, since the 4.18 HWE upgrade failed on the compilation of the ZFS-SPL module (another bug report).
I remember that after trying to repair the system by running update-grub from another system on sdd5, the btrfs sdc5 boot entry has not been detected anymore by the Linux 4.15 update-grub.

The sdc5 has been used before by an ext4 partition with Xubuntu 18.10 and I restored that partition.
Tomorrow I could try to reconstruct the problem in a Virtual Machine and I could send you that vdi or vdmk file with the boot error. Else I retry the install.

I have 3 disks and a SSD and they are:
- sda of 500GB (3.5") has 2 ZFS partitions one for virtual machines and one for data.
- sdb of 320GB (2.5") has 2 ZFS partitions one for virtual machines and one for data.
- sdc of 1 TB (3.5") has 5 partitions, one fresh install of btrfs for the system (Xubuntu 18.04.2 on sdc5), one swap (sdc6), one zfs for virtual machines and one zfs for data and the last 500GB is zfs used for archives.
- sdd of 128GB (SSD) has an extended partition with 3 ext4 system partitions (Xubuntu or Ubuntu Mate, one 18.10 and two 18.04.2) and one ZFS Log partition for data. The SSD has three primary partitions, two for ZFS Cache and one for ZFS Log for virtual machines. All HDD ZFS partitions are lz4 compressed.

I have that many system partitions since the main one uses Virtualbox and the two others contain VMware and QEMU/KVM. I keep the sdc5 partition to be independent of the SSD, if needed. I lost one SSD in the past.

I also use btrfs with Peppermint 9 (Linux 4.15) on a Pentium 4 HT with two 40GB IDE HDDs. I installed it on one disk with a root and home physical partition and balanced both partitions over the two HDDs. Afterwards I compressed both btrfs volumes using LZO. That 15 year old PC booted in an amazing 45 seconds instead of a couple of minutes, Partly caused by striping and partly by the compression. That did give me the idea to replace all ext4 system partitions by btrfs LZO compressed partitions to gain say 33% in boot time :)
The first try-out failed.