You may chainload PCLinuxOS from OM too in order to avoid this.
The above command are form OMV, I don’t know how exactly work PCLinuxOS.
You real setup is different from the vbox one. Because the command I suggested to you doesn’t write a the beginning of the disk (a.k.a MBR) but at the beginning of the OMV root partition this should not affect any other installed SO.
However test it in a vbox is good practice.
" Because the command I suggested to you doesn’t write a the beginning of the disk (a.k.a MBR) but at the beginning of the OMV root partition this should not affect any other installed SO."
But that it exactly what I doubt.
Because in the Oracle-Vbox setup I installed OMV in the sda2 partition where in sda1 there is PCLinuxOS. And going through the installation procedure, I did the installer let its (OMV) bootloader install in its / partition. But, I discovered after rebooting, the system booted right into “rescue grub”.
So, I do not know what is happening but I am sure that there is something not going the way it should be.
I still have to look at that possible microcode-intel package problem ben79 wrote about earlier but did not yet got the time to do so…
When you did the large update grub2 package also was updated. As a result of the installation of the new grub2 package this installed grub in you system (into MBR) but it failed to detect where you installed the previous one (into / partition).
Please could you post here the output of the following command (which should be the place where grub2 package installs grub)?
df -h /boot |(read; awk '{print $1; exit}'|sed 's/[[:digit:]]*$//')
In you real machine you have a different scenario because grub into MBR is the PCLinuxOS one and the command I wrote installs grub2 at the beginning of partition (it because the latest character is the number of partition: if you omit the number you install grub into MBR of the specified disk, i.e. /dev/sdc, and in this case you will break your system as your vbox).
Your response leaves me quite puzzled.
In what OS do I have to run:
df -h /boot |(read; awk '{print $1; exit}'|sed 's/[[:digit:]]*$//')
In the Oracle-VBox version? But if so, how can I do this when OM causes the “rescue grub”?
Or do I complete misunderstand things? Moreover, as I already wrote in one of my last responses, I uninstalled OM in my VBox after repairing the MBR there. But I am of course willing to reinstall OM again to do some tests.
BTW, I still have OM left uninstalled om my “real world” pc. 
You’re rigth, I mean in vbox. About rescue grub you may try to insert the boot command sequence manually. I put an example here (note here is the syntax for grub2, you may retrieve the syntax for grub from one of the above posts)
kernel (hd2,msdos1)/boot/vmlinuz-4.13.5-desktop-1omv root=/dev/sdc2 ro
initrd (hd2,msdos1)/boot/0a7b0bd3dce949e68abe3463506cb0ce/4.13.5-desktop-1omv/initrd
because paths may be wrong you have to discover the right ones. You may activate path completion by using TAB exactly as in a terminal emulator.
I’m closing this discussion. OP I think has decided to not use OpenMandriva for now as it does not work with his (legacy) boot method. Please start a new thread if the issue occurs again or a similar issue occurs.