Tbh I'd just image your sd card (just in case) with eg win32diskimager or usbit (assuming you're using windows) and then not worry about it until it happens. I'm pretty sure marshmallow won't convert an sd card without a user input telling it to.
As it turns out, on booting the android system, it notifies me every time that the external sd card is corrupt, even though it mounts the LibreELEC system
(vfat) partition, but not the ext4 partition. So it also offers to 'fix' the external sd card by formatting it. Apparently it sees the partition as not ending on a cylinder boundary:
# fdisk -l /dev/block/mmcblk0
Disk /dev/block/mmcblk0: 31.1 GB, 31104958464 bytes
4 heads, 16 sectors/track, 949248 cylinders
Units = cylinders of 64 * 512 = 32768 bytes
Device Boot Start End Blocks Id System
/dev/block/mmcblk0p1 * 33 16417 524288+ c Win95 FAT32 (LBA)
Partition 1 does not end on cylinder boundary
/dev/block/mmcblk0p2 16449 949248 29849600 83 Linux
Partition 2 does not end on cylinder boundary
That's a bit of a nuisance but of course the solution is to remove the external sdcard.
As an aside I would imagine I can safely delete the 'Android' and 'LOST.DIR' folders (I think) the android system wrote to the external sd card. See below:
# ls -l /storage/10D4-E16E/
total 135336
drwxrwx--x 4 root sdcard_rw 8192 Dec 31 2014 Android
drwxrwx--x 2 root sdcard_rw 8192 Dec 31 2014 LOST.DIR
-rwxrwx--x 1 root sdcard_rw 129531904 Oct 15 12:49 SYSTEM
-rwxrwx--x 1 root sdcard_rw 48 Oct 15 12:49 SYSTEM.md5
-rwxrwx--x 1 root sdcard_rw 1033 Oct 15 12:49 aml_autoscript
-rwxrwx--x 1 root sdcard_rw 159744 Oct 15 12:49 dtb.img
-rwxrwx--x 1 root sdcard_rw 37648 Nov 1 19:59 dtb.img.new
-rwxrwx--x 1 root sdcard_rw 8790016 Oct 15 12:49 kernel.img
-rwxrwx--x 1 root sdcard_rw 48 Oct 15 12:49 kernel.img.md5
-rwxrwx--x 1 root sdcard_rw 2492 Oct 13 15:48 remote.conf
-rwxrwx--x 1 root sdcard_rw 668 Oct 15 13:49 s905_autoscript