Downloaded songs always delete from SD card

[Diagmel]

Hello, I am new to this forum.

So I have a lot of music that I have been saving on my Galaxy S5 (android version 6.0.1) for a while now, but it got to the point where I don't have enough space to download any more. Easy solution; I bought a 128gb SD card.

The problem is, everytime I download all 600 or so of my songs onto the SD, Spotify lags a lot and untimately crashes, signs me out, and deletes the whole cache so I need to re download them. 

I have tried a lot of different things, including uninstalling, and deleting cache many times. I even logged myself out everywhere, and changed my Spotify and Facebook passwords in case somebody else had access to them. I used to have the problem where 

Why does it only temporarily save onto my SD card? Is there a way to save in two locations at once?

It's annoying because I pay 10$ per month for this, I'm considering unsubscribing and just downloading all my music the old fashioned way.

At this point I'm going to try downloading less songs at once, and see if the problem still happens.

Maybe my SD card is the causing the issue?