It occurs due to the power-saving design of computers running Windows 7 or later versions. Whenever an external USB device is left idle on a Windows 7 computer or later versions, the system will automatically remove it to save power.
In these circumstances, please re-plug your flash drive for it 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 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 tap "OK." Reconnect the flash drive.