Boot the Debian installer and get to a recovery console prompt.
Run lsblk (8) and determine the device node for your ~500 GB drive (it should be /dev/sda, but you should confirm this):
https://manpages.debian.org/jessie/util-linux/lsblk.8.en.htm l
# lsblk
Then run parted (8) to see how the disk is partitioned:
https://manpages.debian.org/jessie/parted/parted.8.en.html
# parted /dev/sda u s p free
Take a photograph of the screen with the two commands and their output, upload, and reply with the URL.