AFAIK, the Primary System partition is the one that contains the Windows directory of the system you are currently running on.
If you choose to boot off of the HDD based Windows, that becomes the Primary System, if you choose to boot off of the SSD based Windows, then that becomes the Primary System.
Makes perfect sense to me, after all, if you specified that the hibernate file for the SSD is on the HDD, what stops it getting overwritten if/when the HDD based Windows goes into Hibernation?
What does Disk Management show for the characteristics of the SSD when you've booted off of it?
As stated here
, the bootloader doesn't have the capacity to deal with all the various drive configurations, (RAID, etc), it contains a minimalistic filesystem driver, the driver for anything more complex is contained within the hibernate file - so what do you do?
You make sure that the bootloader can load the hibernate file and to do this you put it where it's guaranteed to be able to be read - on the drive containing the current OS.