Almost spot-on
@Hadron .... I am right now typing from an S5 (klte) and when it was completely stock (standard firmware, android 6.0.1) I was able to in settings -storage -(select SD card) -format as internal
Perhaps if it is not there where I stated then perhaps it wasn't included in your particular firmware (Samsung s5 has about 10 types.... Klte, kltef, kltexx etc etc etc) google search will tell you! E.g ... 'Kltexx format storage' etc
But as
@Hadron stated there are a few options:
A. Clever apps like apps2sd and theres another 3 or 4 decent ones that will pop up in related on app store.
B. There is an ADB command script that will allow use of all of the SD or just part of the SD so some can still be portable storage! (You will need a PC, download ADB and 10 minutes of time)
C. Make sure you you have the latest firmware update on your phone! Check on settings - phone -update AND online also!
D. Download Odin to PC.... Flash a custom ROM and finally RELEASE the performance and potential of this great old phone!!! You will be glad you did!!!
My S5 was feeling dated and sluggish now it is running better than my friends S6!! I was running Resurrection Remix (known as THE most customisable android version!) 5.8.5 final (android 7.1.2) which honestly COMPLETELY changes the phone for the better! SOO much better!! Goodbye 160 stock (bloat) apps... down to about 10 apps afterwards to run your phone! Cosmic OS is also amazing too!! Now I'm running Resurrection Remix 6.0 unofficial build (latest android 8.1) which is amazing also but has a few bugs the older final version doesn't have.
All of the custom ROMs for the S5 (and there are many!!) Allow formatting of storage also!
It is very simple to flash a custom ROM on Samsung thanks to Odin!!
Just make sure you flash a custom recovery first (TWRP - 3.2.0.1 is the best) so that any mistakes can be undone!!
Will take you thirty seconds on the computer to start the process, followed by a 15 minute wait whilst it flashes on PC, DONE! ...now just a 10 minute wait for it to turn back on! Hello fast S5!! Hello quick tiles! Hello picture in picture mode! Hello adaptive display! Hello world!