adb shell
sm list-disks
// HERE YOU GET YOUR DISK ID, SOMETHING LIKE "disk:179,64" - REMEMBER THOSE NUMBERS
sm set-force-adoptable true
// IN NEXT LINE, SIMPLY PUT THOSE NUMBERS AFTER "disk:" AND ALSO AFTER WORD "mixed" TYPE PERCENTAGE OF SPACE LEFT AS EXTERNAL, SO IN MY CASE:
sm partition disk:179,64 mixed 60
// IT TAKES TIME. BE PATIENT. WITH THIS LINE I TRANSFORMED WHOLE EXTERNAL SD INTO 40% OF INTERNAL AND 60% OF EXTERNAL
sm set-force-adoptable false