Formatting your hard drive as "case sensitive" was a mistake. Though I doubt that it is the source of your problem. You can fix it following these instructions:
Itu2019s tedious to convert these partition types, but not complicated.rn
www.macworld.com
Let's try something to try and fix your main problem that will be fairly quick, free, and easy; and see if we get lucky...
Download:
Permissions Reset (free)
Permissions Reset 2
In some instances, Permissions Reset can fix incorrectly set permissions in an instant. Use it on your external hard drive and let us know how things go.