Comment by eichin

Thinking of it in terms of namespaces might help; it's not that the drive is special, it's that there's a view that starts from / and one disk filesystem happens to be dropped there and others are dropped elsewhere; with something like initramfs there aren't any drives on /, just a chunk of ram, though you usually pivot to a physical one later (many linux-based embedded systems don't because your one "drive" is an SD card that can't handle real use, so you just keep the "skeleton" in memory and drop various bits of eMMC or SD or whatever into the tree as-convenient.)