When ejecting a flash drive from an actively running computer, you risk corrupting its files. To prevent this, Windows 7, 8 and 8.1 feature a built-in utility that automatically starts in the system tray whenever you insert a flash drive. This utility notifies you whether it's safe to eject your flash drive, and warns you if the drive's in use.
Safely Removing a Flash Drive
When you insert a flash drive into a computer running Windows 7, 8 or 8.1, an icon reading Safely Remove Hardware and Eject Media displays on the system tray. To unplug your flash drive, right-click that icon, and select Eject USB Flash Drive. If it's safe to eject, a notification displays Safe to Remove Hardware.
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If instead, the utility notifies you that the flash drive is in use, close any files or applications that you are running off the flash drive, and then try again. If a random process still prevents you from ejecting the flash drive, log off of your Windows user account, log back on, and retry.