so for 2 months I was using my external HDD to play games and I have had no problems since then. but last night, as in so suddenly, in the middle of my browsing, i thought of playing a game and as i go into my HDD, suddenly a window saying "Access Denied" and something like I don't have ownership popped. with that, I chose to manually add permissions and ownerships and guess what, it got back to normal operation and the question in my head which is the first of the2 questions I am about to ask here is How? i mean How? i did not even do anything, just normal youtube and facebook browsing. okay so now the problem is gone and the next day I thought of testing my HDD with my Desktop PC and guess what, I can run files from in there normally and without any problems except this.....I cannot transfer files to and from the HDD...it says that I do not have permission to do that which comes to my 2nd question as to how to fix this. is there someone here experiencing this also? anywaaays, Thankks in advance!

Note: I also cannot download files from the internet directly to my HDD, it also says I am not authorized to do it.