grub-probe fails to recognize ext4 partition

[Tobias Koch]

On a recent Bionic Azure image, the command

    grub-probe --device /dev/sda1 --target=fs_uuid

fails with

    grub-probe: error: not a directory.

The reason for this seems to be that grub-probe finds the minixfs superblock magic 0x138F at offset

    00000410 8f 13 3a 00 00 00 00 00 02 00 00 00 02 00 00 00 |..:.............|

then assumes it has found a minixfs-formatted partition and exits with an error when it cannot access it, instead of trying other filesystems first.

I don't know what the ext filesystem has stored at that position, and whether it is straight-forward to fabricate this manually.

To reproduce this, you may launch the Azure Bionic image with urn

    Canonical:UbuntuServer:18.04-LTS:18.04.201906271