I couldn't transfer music files from my Samsung Galaxy S9+ to my MP3 player using theISB cable and connector. It's only charging the MP3 Player but it wouldn't let me switch to phone to carry out the transfer. I've done it before and it was ok, but now it won't allow me. I don't know what's wrong. Each time l click on phone, it says couldn't switch and it remains on USB Connect which only charges the MP3 Player.
Same thing happened on my S9+ even without the usb flash drive attached to the connector. Been scouring the internet for a solution but this is the first post I'm coming across. I've got an Huawei P20 mate and the same thing has happened so I think this is down to the recent updates. I've four different connectors and it's ihe same. Can't switch from usb device control to phone control and on the P20 mate it just switches back and disappears. Any solution is very much welcome.
This is an Android 10 security feature, a good comparison for which is putting everyone in prison as a crime deterrent! The excuse for adding it is complete, feeble, thinly veiled control over anonymously putting files on a mobile device.
Sheesh, it's not like I need to save money. I think I'll get a secure phone that runs Linux with a hard SELinux policy. Their only about 900.00USD. And they have a googlized VM to interface with the play store. It strips out the objectionable parts of Android apps, converts them to run under the Linux kernel, so you still have all the apps Android has!
I think i have have a possible answer to this question. I have done some minor experiments on this matter and I am sharing these results here today.
Firstly if your using an OTG cable or adapter then it wont allow you to change it's mode. This is because the OTG cable/.adapter is configured and hard wired to control the USB function. When it says USB is controlled by "Connected device" it refers to OTG cable/ adapter having the control and NOT the device plugged into the OTG cable or adapter.
If you have any doubts then do the following experiment, plug your standard data cable into a computer and the phone and check the settings it will say "This Phone" under USB controlled by. Now leave that screen up and keep your display active and only plug in a OTG cable / adapter with nothing plugged in to it. You will see it now says that USB is being controlled by "Connected device". This is the actual hard wiring of the OTG cable/Adapter that has the control.
The phone needs to see your MP3 player as a memory storage device. Go to my files on your phone and see if your MP3 player lists as a USB device in the list of devices. If so then select your files and then choose to copy to the USB storage device ( in any folder you require on that device).If your phone doesn't see a USB storage device (phones won't always show the name of the actual device), Then you could try a new OTG adapter or cable and see if that's the problem. You can also connect the MP3 player to a computer and verify it can read the device ( to rule out that the MP3 player is not got a faulty USB port).
This is my opinion and I hope it helps anyone with the same issues as the original poster.
stay safe and well everyone