if you haven’t solved your issue yet. I am in the same boat and would like to use a USB-HDD as storage for data and media which again will be shared via some sort of media server (some DLNA thingy + nextcloud).
On my research I came across this: https://github.com/home-assistant/operating-system/blob/f4f6908c80ac19128aa0a1de0377e0954c5902ee/Documentation/partition.md#data
Haven’t tested it myself yet, but seems to work fine according to other comments.