Since its beginnings, crappy Android has made it difficult for users to achieve the very basic feature of mounting the SD storage of the phone as a USB drive, when the phone is connected to a PC by cable. You can see the phone and SD under My Computer, but not as a USB drive that apps can access. My workaround to this embarrassing shortcoming has been an app that did just that but that only works on a rooted phone - which my new Marshmallow Android phone is not. Is there any other workaround?
As you said, it's possible only if your device is rooted..
Google no longer supports USB Mass Storage Mode due to several reasons, since JellyBean.
However, there are softwares for PC that virtually shows & provides access to the device (connected in MTP mode) as a Removable Storage USB Drive, like for example, MTPdrive (as @beeshyams commented); no root access required in device. But it's just a virtual round-about connection, hence it'll be slower than UMS & even MTP mode..
Or try WiFi USB Disk - Smart Disk, which also virtually shows your device as a USB Drive (through WiFi). Or you can do that yourself using file servers by mapping a network drive to your device.
To get your device's memory card actually mounted as a real UMS device, the only way is to root your device..
There are many root apps that would help you enable UMS. Some of them are:
Note: Some apps require you to disable SELinux (that is, setting SELinux mode to 'Permissive' by setenforce 0 command)
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