I recently seem to have lost ownership of my external, exFAT formatted USB HDD's. Root owns them now. I recently ran Photorec to look for some deleted files and noticed this issue after that. But I also downgraded my kernel too so I'm not sure if it's that which has caused it. Anyway, Im using Linux Mint 19.1 and I opened my file manager (Nemo) as root and then tried to change the ownership of these HDD's back to my user account via the GUI but it just tells me that the operation is not permitted. If I can't change it as root then how can I actually get these drives back?

Thanks!
Updates:
* Apparently these drives were mounted as Ready Only. I've successfully remounted the drives with Read/Write access.
* GUI & Terminal methods for taking ownership are still returning "Operation not permitted" messages. Root still owns them.