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Open Disk Utility:
Go to Applications > Utilities > Disk Utility.
Select the USB drive:
Insert the USB drive.
On the left side-bar, there should be an External section. The USB drive should be listed under this section. To verify that you have correctly identified the drive, you can unplug and re-insert the drive while observing if the listing disappears and reappears.
Select the drive from the left side-bar.