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You can add music to a traditional MP3 player on both Windows and Mac computers.  To add music to an iOS item such as an iPhone or iPod, you'll need to add the music to iTunes on your computer and then synchronize your iOS item with iTunes. If you want to add music to an Android smartphone or tablet, you'll need to use Google Play Music or a USB cable. If you're on a Windows computer that uses Windows Media Player, you can add your music via Windows Media Player instead (this won't work for Apple items like the iPhone). Select the music by dragging your mouse cursor over it, then press either Ctrl+C (Windows) or ⌘ Command+C (Mac). Plug one end of the MP3 player's USB cable into your computer, then plug the other end of the cable into your MP3 player. If you're using a computer that has USB-C ports instead of USB 3.0 ports, you'll first need to buy a USB 3.0 to USB-C adapter and attach it to your computer. This process will vary depending on your computer:   Windows — Open {"smallUrl":"https:\/\/www.wikihow.com\/images\/b\/b1\/WindowsFileExplorer.png","bigUrl":"\/images\/thumb\/b\/b1\/WindowsFileExplorer.png\/30px-WindowsFileExplorer.png","smallWidth":460,"smallHeight":460,"bigWidth":"30","bigHeight":"30","licensing":"<div class=\"mw-parser-output\"><p>I edited this screenshot of a Windows icon.\n<\/p><p>License: <a rel=\"nofollow\" class=\"external text\" href=\"http:\/\/en.wikipedia.org\/wiki\/Fair_use\">Fair Use<\/a><br>\n<\/p><\/div>"} File Explorer, click This PC, and double-click your MP3 player's name in the "Devices and drives" section. If your MP3 player's window opens automatically, skip this step.  Mac — Open {"smallUrl":"https:\/\/www.wikihow.com\/images\/1\/1d\/Macfinder2.png","bigUrl":"\/images\/thumb\/1\/1d\/Macfinder2.png\/32px-Macfinder2.png","smallWidth":460,"smallHeight":431,"bigWidth":"32","bigHeight":"30","licensing":"<div class=\"mw-parser-output\"><p>I edited this screenshot of a macOS icon.\n<\/p><p>License: <a rel=\"nofollow\" class=\"external text\" href=\"http:\/\/en.wikipedia.org\/wiki\/Fair_use\">Fair Use<\/a><br>\n<\/p><\/div>"} Finder, then click your MP3 player's name on the left side of the window. You may also be able to double-click the MP3 player's name on the Desktop. Depending on your MP3 player, you'll either find this folder inside the initial MP3 player's folder or you'll open the "Internal" or "Storage" folder first.  You may have to open a "Music storage" folder instead. You'll know that you have the correct folder when you find one that has all of your MP3 player's song files. Once you're in the "Music" folder, press Ctrl+V (Windows) or ⌘ Command+V (Mac) to paste in your copied music. It may take a few minutes for your music to finish copying into the "Music" folder. Ejecting your MP3 player before disconnecting it from your computer will ensure that you don't accidentally corrupt its files:   Windows — Click {"smallUrl":"https:\/\/www.wikihow.com\/images\/7\/70\/Android7expandless.png","bigUrl":"\/images\/thumb\/7\/70\/Android7expandless.png\/30px-Android7expandless.png","smallWidth":460,"smallHeight":460,"bigWidth":"30","bigHeight":"30","licensing":"<div class=\"mw-parser-output\"><p>I edited this screenshot of an Android icon.\n<\/p><p>License: <a rel=\"nofollow\" class=\"external text\" href=\"http:\/\/en.wikipedia.org\/wiki\/Fair_use\">Fair Use<\/a><br>\n<\/p><\/div>"} in the lower-right corner of the screen, right-click the flash drive icon, and click Eject.  Mac — Click the "Eject" {"smallUrl":"https:\/\/www.wikihow.com\/images\/3\/3f\/Maceject.png","bigUrl":"\/images\/thumb\/3\/3f\/Maceject.png\/30px-Maceject.png","smallWidth":460,"smallHeight":460,"bigWidth":"30","bigHeight":"30","licensing":"<div class=\"mw-parser-output\"><p>I edited this screenshot of a macOS icon.\n<\/p><p>License: <a rel=\"nofollow\" class=\"external text\" href=\"http:\/\/en.wikipedia.org\/wiki\/Fair_use\">Fair Use<\/a><br>\n<\/p><\/div>"} button to the right of the MP3 player's name in Finder.

Summary:
Make sure you have the right kind of MP3 player. Copy the music you want to add. Attach the MP3 player to your computer. Open your MP3 player's folder. Find the "Music" folder. Paste in the copied music. Eject and remove your MP3 player.