how do i use a usb stick on my imac

On the menubar in Finder, Finder > Preferences > General tab > check external disk.

The icon should then show on the desktop.

Open Finder and see if you see the USB thumb drive there. If not, open Utilities/Disk Utilities and see if you see it there. It may need to be formated as GUID and Mac OS Extended (Journaled).

Choose Computer from the Finder's Go menu and double-click the drive.

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Plug it into one of your USB ports. Then do the following:

Drive Preparation

1. Open Disk Utility in your Utilities folder.

2. After DU loads select the USB flash drive (this is the entry with the mfgr.'s ID and size) from the left side list. Click on the Partition tab in the DU main window.

3. Under the Volume Scheme heading set the number of partitions from the drop down menu to one. Click on the Options button, set the partition scheme to GUID then click on the OK button. Set the format type to Mac OS Extended (Journaled.) Click on the Partition button and wait until the process has completed.

4. Select the volume you just created (this is the sub-entry under the drive entry) from the left side list. Click on the Erase tab in the DU main window.

5. Set the format type to Mac OS Extended (Journaled.) Click on the Security button, check the button for Zero Data and click on OK to return to the Erase window.

6. Click on the Erase button. The format process can take up to several hours depending upon the drive size.

Then you most likely have a defective flash drive. If this is one of those flat stick with the visible gold contacts, then try turning it around in the USB port.