Cannot copy files from internal storage to external SD

Question

Device: Moto X Play Stock (6.0.1) with external SD as portable storage

• Prior to rooting (recently), transfer both ways (internal and external SD and vice versa was not a problem)

• After, rooting, it is one way- I can copy files only from external SD card to internal . I am assuming that external SD is not the problem since

  • Device shows external SD, so do various file managers- ES Explorer, Total Commander and all of them fail to do this

  • External SD recognized on laptop and copying / writing is not a problem

• In Marshmallow, when you need to copy to external SD, you get a notification as below, and in the second pic you expect to see the external SD card listed and you accept- this doesn't happen, there is neither external SD shown nor option to select

• Rooting couldn't or shouldn't have caused this. Further, TWRP option of not allowing modifications to system was chosen

• I didn't mess with any system tweaks,except for installing Xposed and a few modules (Edit Uninstalled Xposed and behavior is same)

What could be causing this and how to fix it?

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Note:

1. I do have access to Mint Linux but practically zero knowledge of commands. If Linux helps in solving this , please be explicit about instructions

2. There a couple of questions similar but not the same problem and do not help

Answer 1

Solved it by shutting down device, removing external SD card, rebooting a couple of times, inserting external SD and starting the device

It worked but clueless as to what was wrong and what fixed it

I am hoping for a proper explanation/solution to show up