These are both dedicated drives...no files whatsoever on them outside of BI clips. C: drive is dedicated OS drive and BI program files with DB on them. It's a 256G SSD drive with over half of that space free.
Mine is setup to delete from the stored directory after 2tb, which it is always close to. So the storage isn't the issue. I deleted the db directory and rebooted and it is working again. Just a reboot didn't fix it, so something was wrong with the db.
I haven't changed anything in my setup for months. It seems the cameras that were saving to "stored" stopped, while the one saving to "new" and being moved to stored was still working, so the drive wasn't the issue if blue iris was able to move the files from new to stored.