Add .dropboxignore directory to exclude folders without using selective sync

Christopher N.1, you recommend storing the files outside of the Dropbox folder; this is exactly what I want to do, but every time I attach an external drive or card reader to my pc, the files automatically synch to DB, so how do you pre-set a folder to not synch to DB?  It is sort of the reverse of synching, I want the files on PC, not the cloud.  Had a similar issue with OneDrive; 1Tb hard drive and 20gb OneDrive account.  When OneDrive was full (with files I didn't want there) I cleared some space and lost years of photos from my hard drive.  I'm terrified of it happening again, how do you protect files on (multiple) hard drives from the capricious whims of these cloud accounts?  I don't want or need to subscribe to an infinite amount of cloud storage, but sometimes it feels like these apps are just a legitimised form of ransomware; counter-intuitive and designed to put your data in peril unless you cough-up for the paid service.