Indeed. This is the first report that I can remember of X1 not spotting internal drives. Which is why I suspect something wonky on your system that is confusing X1.
X1 won't index files on extra drives unless you tell it to do so. Have you been into Menu->Data Sources->Files->Options to confirm that this drive is missing?
I assume that the drives are not "Hidden" and that they show up properly in Explorer and in Device Manager.
If this is a recent purchase and none of the above applies, you should raise this with support. They might find it an interesting case to investigate.