The Finder application on Macs running OS X Yosemite enables you to transfer files on and off your computer. When you connect an external hard drive to your Mac, it appears on the Devices menu in Finder. Copy and paste, or drag and drop, items onto the hard drive as you would to any other location on your Mac.
Connect the external hard drive to your computer using its provided cable. The cable used will vary depending on the make and model of the hard drive. Most mainstream hard drives use USB cables.
Click the "Finder" icon on the Dock to open the Finder application.
Select the folders or files you want to transfer onto your external hard drive. Hold down the "Command" button to select multiple items simultaneously.
Press "Command-N" to open a second Finder window and then select your external hard drive on the list of devices displayed under the Devices heading on the left side of this new window.
Browse to the folder on your external drive where you want to store the files or folders and then drag and drop folders and files from your Mac's Finder window into the external hard drive's window. A status bar appears on your screen indicating progress. Wait until the full transfer completes.
Click the "Eject" button to safely remove the hard drive when you're finished.
You can also use copy and paste functionality on your Mac to manually copy and paste files from the computer onto the hard drive. Click “Edit” on the main menu at the top of your screen and select “Copy” to copy items to your clipboard, or just press “Command-C.” Click “Edit” on the main menu at the top of your screen and select “Paste” or “Paste Item” to transfer the items onto the hard drive at your current location -- or just paste by pressing “Command-V.”
If your external hard drive is new and has never been connected to a Mac before, you may need to format it in order to make it usable with your computer. Some drives may also need to be formatted if they’ve previously been connected to a Windows PC. If your drive falls into the latter category, back up any data you want to keep before proceeding.
To format a drive in OS X Yosemite, connect the drive to your Mac and open the Finder application. Click “Go” on the main menu and select “Utilities.” Click the “Disk Utility” icon to open Disk Utility. Select your hard drive from the list of devices, click the “Erase” tab located along the top of the window and then select “Mac OS Extended (Journaled)” on the Volume Format drop-down menu. Enter an identifying name for your hard drive in the provided field and click “Erase” to format the drive.