LVM PV on second disk only activated in some circumstances

[Xen]

I have someone of an unusual setup, call it experimental.

I have two disks.

One contains a partition with a PV that embeds two other PVs.

One of those two other PVs is the container of a RAID 1 mirror for the 
first disk.

To get to those mirror volumes you'd first need to activate the 
containing PV.

Now when I boot from the first disk (Grub compiled from Savannah a few 
months ago) using a grub menu located on one of the mirrored volumes 
which is also, in part, on that first disk, and I enter the Grub command 
line...

And I execute "ls", I get a full list of all the VGs that are on both 
disks, meaning it has found and activated the "container" PV on the 
second disk, but not only that, it has activated (or found) the PVs 
inside of that, one of which is not mirrored and located only on that 
disk.

So these two disks: call them sda and sdb.

sda contains a PV with volumes (one of which is my boot volume) which 
are one half of a RAID 1 mirror.

sdb contains a PV with this:

coll/msata-lv
coll/data-lv

Those two are PVs, the first contains one half of:

msata/root
msata/boot

The other one contains:

data/<several>

These "data" lvs are not related to sda and their activation thus 
requires 2 steps, yet they ARE found when I boot with Grub from this 
sda.

---
Now when I boot from Grub located on some USB stick (both image and 
modules/cfg) and I perform "ls" I only get:

(stick/root) (stick/boot) (msata/root) (msata/boot)

But not:

(data/first) (data/second)

That I got from booting the sda disk.

It has the same version of Grub.

It does not require "coll" to be activated to find its "boot" volume and 
that is the main difference?

How do I force "coll" to be activated?

It is basically sitting on /dev/sdb4

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However when I disconnect the sda disk, nothing gets found at all. The 
point is that....

Situation A: boot from SDA: finds everything: (coll/msata-lv) 
(coll/data-lv) (msata/root) (msata/boot) (data/first) (data/second)

Situation B: boot from stick with SDA connected: finds only:

(msata/root) (msata/boot) (stick/root) (stick/boot)

Situation C: boot from stick with SDA disconnected: only finds:

(stick/root) (stick/boot)

So it is ONLY when I boot from SDA that SDB4 even gets activated.

When I boot from Stick SDB4 does not get activated, although it should 
be found all the same (no Issues there I believe).

If I boot from Stick with SDA then it only finds and activates volumes 
on SDA (and stick)

If I boot from Stick without SDA then it only finds and activates stick 
itself.

In all cases SDB is connected.

What is the issue here and how can I force SDB4 to be activated?

TLDR: PV on second disk only gets activated when (apparently) referenced 
by PV located on first disk because/when that first disk contains Grub's 
boot files. Booting from anything else doesn't get the second disk 
activated. Booting from something else still finds volumes on first 
disk. But not second disk. No outer PV, and no inner PV.