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The easiest way to do this in any open window is to hold down Ctrl and press S. It's the rectangular, flash drive-shaped icon with a checkmark next to it. It lives in the row of options that are in the bottom-right corner of the screen, though you may first need to click the upward-facing arrow on the far-left side of these options to view it. Doing so will invoke a drop-down menu. This option is at the bottom of the drop-down menu. It will typically say something like Eject SDHC (E:) with your flash drive's name written after the "Eject" prompt. It should slide out. The next time you plug your flash drive into your computer, all of your files should be as you left them.

Summary:
Save any open documents you have on the flash drive. Find the "Eject" icon. Click the "Eject" icon. Click Eject. Gently tug the flash drive away from your computer.