According to what I've read, a 'File Transfer' window should open. Mine just says, 'slow charging'.
1. first check your usb cable
a. your cable could be a "charging only" cable (i.e. only has wires to carry power and no data wires)
This cable could be a
charging-only cable, which doesn't include the
D- cable streams. If you're unsure on the type of cable, try using a different cable that you're sure works.
b. if you are using usb-3, it could be "bad"
USB-2 minimum cable requirements were easier to manufacture because of the lower data rate requirements. Ever since USB-3 lots of USB-3 cables don't meet "spec" and therefore can not work or damage your phone. Make sure you have a brand that you know is good.
2. After checking the cable, you need to check your phone's settings. make sure it is in "file transfer" mode and NOT "charging only".
From Android 6.0 onwards
File transfer is disabled by default when a cable is connected to the device, mainly due to malicious USB hubs which pretend to be normal charging ports, but instead copy data from your device whilst charging.
To enable File Transfer Mode
- Turn on your device and connect it to your computer (using a cable which supports data transfer)
- Unlock your device and open the notification labelled
USB charging this device- you may need to swipe down from the notification bar
Transfer Filesfrom the menu that appears.
- You should be able to access your device on your computer.
3. try a different USB port on the PC
I tried a bunch of cables on one USB port but nothing was working. So I tried a differnt USB port and it all worked again!
4. lastly. Try... restarting the phone... restarting the computer.
Some might say this is the first thing to try.
Make sure your wire supports both charging and data. If it does then on the phone go to Settings->Storage->->3 Dots-> USB Computer Connection-> Change the mode from Charging Only to MTP or USB Mass Storage. If neither of these work then make sure you have drivers installed for your device on your computer.
Generally, whenever you connect the USB the phone is in USB charging mode by default.
To change to any other mode (file transfer mode in your case), go to the notification pane where a notification would tell you that usb is connected and charging. Tap on it and select the MTP or file transfer mode.