From a quick look, it is on Bandcamp, for those that would rather have it DRM free (FLAC, MP3 and derivatives):
https://garoad.bandcamp.com/album/va-11-hall-a-prologue-ost-sounds-from-the-future
It's also on ITunes (only AAC and MP3), but worth noting getting to the store page is a bit cumbersome even with the link:
https://music.apple.com/us/album/va-11-hall-a-prologue-orignal-soundtrack-sounds/1190726465
Is your drive where you install games automatically mounted by the system? In case something changed in your system, does it have the same path as Steam expects it to? And is the drive a separated storage? And though it may sound like a stupid question, I think it's important to ask also, are you sure it's on the storage you think it is?