How can I make existing permissions apply to new files?
That Terminal command only aplies to existing files as you might have guessed. And there is no modification to it so that it will set the permissions of future files right. It doesn't work like that.

I think you might have a problem with your disk's ownership. Through your Mac's view, you don't owe that disk partition. You must set that right.

And if you ask why the Terminal command works when I don't owe the disk, it's because of that sudo keyword in front of it. sudo initiates the most privileged user in a UNIX/Linux system, which is the root user, that it can pretty much do whatever it wants. Even if you're the admin user, you can't do everything on your OS; you must switch to the root user. It's basically a safety system in place.

Anyway, in order to set your disk's ownership right,


open a Finder window,
right-click to your external drive,
select Get Info
at the bottom, there must be a Sharing &amp; Permissions section. Check to see if you are listed there. Otherwise add yourself to the list.


Also, besides your current problem, why did you partitioned the SSD into two? You said you did it not to lose your data when you format the OS but when a new Mac OS comes, it's exchanged with the old one when all your personal data stays intact. Besides a Time Machine backup would be much safer if you don't want to lose your data.

Frankly, it's a personal choice and when you look at it as bits and bytes nothing really changes, but keeping the SSD as a whole would be much neater.