this does kinda sound like a hardware issue, the fact it only happens with a specific program is quite common, msfs uses a Lot of disc space And a Lot of temporary disk space (much more than other typical games)
how big and how full is this ssd? ssd’s treat temp space much differently than standard platter drives.
if ssd’s are too full this can be a serious problem (not to mention it will wear them out in a hurry)
the drive is only one possibility here but it may be the most likely (the offline issue suggests more than one problem is randomly happening with “Now suddenly its working…” random failure kinda suggests hardware, software tends to be more consistant)
your having 2 separate and odd issue’s, neither of which have been seen commonly that im aware of
this strongly suggests its something local, ‘hd is missing’ again suggests hardware or drivers
all that does not mean the hardware is bad, it could simply be a driver conflict but these are the kind of things i would start checking if it was me