To clarify - there is no separate drive to hold the operating system. In both the SSD version & HDD version, the OS is stored on the primary drive.
The SSD has been obsolete for a long time now & just recently we exhausted our stocks of the 2TB HDDs.
We have pretty much everything else likely to fail I think. However, drive failure can manifest itself in many different ways, so it can be hard to diagnose without having the unit on the bench.
Regards
Neil.