My MacMini Late 2012 has a 1Tb external USB drive, which I use as the main data store (the internal 256Gb SSD being too small).
I have been running parallels on this Mac for years and both native Mac drives were visible… I think it listed each twice in Windows Explorer for some reason.
Just today I really need to access large (several GB) files but now the external drive is not visible (of course). I found this but it doesn't help: http://kb.parallels.com/en/118147
MacOS 10.10.5, Parallels 13.
Take your drive to another computer and Cloud your files.