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You'll usually find the USB port on the sides of the housing (MacBook) or on the back of the monitor (iMac). If the cable uses a USB 3.0 connection, it should only fit one way. USB-C connections can be inserted in any way that fits the port. The other end of the cable will usually be a proprietary connection that only fits one port on the external hard drive. Click the Spotlight icon, which resembles a magnifying glass, in the top-right corner of the screen. Type in disk utility, then double-click Disk Utility when it appears in the search results. The Disk Utility window will pop up. In the upper-left side of the Disk Utility window, click your external hard drive's name. It's a tab at the top of the Disk Utility window. Doing so prompts a pop-up window. You'll find it in the middle of the pop-up window. Click one of the following options: Mac OS Extended (Journaled) Select this option if you just want to use your external hard drive with a Mac computer. ExFAT Select this option if you want to use your external hard drive with both Windows and Mac computers. It's at the bottom of the window. Doing so will prompt your Mac to begin formatting your drive. Once the formatting process is complete, you can proceed with using your drive as you please.

Summary:
Find your Mac's USB port. Plug one end of the external hard drive's cable into the USB port. Plug the other end of the cable into your external hard drive. Open Spotlight . Open Disk Utility. Select your external hard drive. Click Erase. Click the "Format" drop-down box. Select a file system. Click Erase. Click Erase when prompted.