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Are you trying to connect and use an external hard drive, connected by USB? Am I understanding correctly?
Do you still need help with this?
Hi my name is***** get started. If the USB flash drive is not "working" it means it probably just needs to be in the correct format for your imac to read it or see it. Just make sure that this is a new flash drive or one that can get erased as all files on the flash drive will be erased once you format . Below are the instructions but I can always remote into your computer if you are not comfortable doing these steps your self. Just let me know by replying to this message. However, if you would like to try it out, here is what you need to do:
This will format the entire USB flash drive and then you should see it appear on your mac's desktop, look for the name you called it.
If you do not see it appear on your desktop you might not have the proper finder preferences set.
If that is the case do the following from your desktop:
In the Finder, choose Finder > Preferences.
Click General, then select the items you want to see on the desktop.
For example, if you select “external disks” then yo will see the USB flash drive.
Then close the finder preferences.
Let me know if this helps. If so, please consider to rate my service. Also, let me know if it doesn't work as I can further assist.