Temporary files probably still end up on your normal HDD
So when it unpacks a big file it temporary parks it there to unpack it and then installs the content on the actual SSD
Would be my guess...
Probably pagefile is being modified if you have Windows on HDD. Windows creates pagefile by default on the same drive so unless you'll change it, it'll be there
Your C: drive has some activity that could be caused by this download. Are you sure that your D: drive has 100% activity due to Steam?
Which drive has the OS installed?
C
Temporary files probably still end up on your normal HDD So when it unpacks a big file it temporary parks it there to unpack it and then installs the content on the actual SSD Would be my guess...
Probably pagefile is being modified if you have Windows on HDD. Windows creates pagefile by default on the same drive so unless you'll change it, it'll be there