This is due to the power-saving design of computers with Windows 7 or later versions. Whenever an external USB device is left idle for some time on a computer with Windows 7 or later versions, the system will automatically remove it to save power.
In these circumstances, you just need to re-plug your flash drive to be recognized by the system again, or disable the "USB selective suspend setting" to stop this from happening.
Please refer to the following steps to disable the "USB selective suspend setting":
1. Open “Control Panel” and select “System and Security”.
2. Select “Power Options”.
3. After entering the "Power Options" menu, find “Preferred plans” and click "Change plan settings".
4. You will see the time to turn off the display and the time to put the computer to sleep. Click the button "Change advanced power settings" below these options.
5. Find “USB settings” in the window. Expand the menu and change the "USB selective suspend setting" to “Disabled”, and then tap “OK”. Reconnect the flash drive.