Ventoy

best multi booting usb tool I have found 

Ventoy

I use a SanDisk extreme 1tb usb drive and with this tool i can do so much.

goto downloads /unzip then goto ventoy2disk.exe

choose your usb drive of choice.

then just drag your isos to the empty drive. it should be the first usb partition ie :/e :/f would be :/e

I did try to use this drive as a backup drive and splitting the partition in half again and that did not work. 

If you want to learn more you can try techno tim software engineer

https://youtu.be/z1FyoCswwAc