After you install iTunes on your computer and set up your iPhone, iPod Touch or iPad, connecting the computer and device is usually straightforward: open iTunes, plug in your device with the USB cable and you're set. When your device disappears from iTunes or repeatedly vanishes and reappears, however, you might have a faulty cord or another problem.
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Device Repeatedly Disconnects and Reconnects
If your iOS device shows up in iTunes when you plug it in to your computer, but then repeatedly disappears and reappears, you probably have a bad USB connection. You might also notice a repeated noise from your device or computer speakers each time the link fails and reconnects. Try another cable, if you have one, or try another USB port on your computer. Until you can replace a malfunctioning cable, set your device down when you sync to avoid jiggling the cord.
A malfunctioning port on the device itself could also cause the connection to fail repeatedly. If using a different cable and different port on your computer doesn't help, your device might need repair.
Device Disappears and Doesn't Reappear
When your device vanishes from iTunes and doesn't come back, try quitting the program, unplugging the USB cord, reopening iTunes and reconnecting the cord. If that doesn't help, reboot your computer and turn your device fully off and on. Don't just turn the device's screen off -- press and hold the Sleep/Wake button until you see a red slider, and swipe it to shut down iOS. Once both machines restart, try the connection again.
Expert PC users: for a shortcut, restart Apple's device service without rebooting your computer. In your computer's Services Control Panel, select Apple Mobile Device, and click Stop the service and then Start the service.
Device Doesn't Allow Access
As a security precaution, iOS 8 keeps track of which computers you connect to. When you connect to iTunes on a different computer, you need to "trust" it before the device will work in iTunes. IOS remembers your trusted computers, so your device won't usually disappear from iTunes, but changes -- such as an iOS version update or a system restore -- can erase the list of trusted computers, causing the device to stop appearing in iTunes.
After you plug in your device, unlock it and tap Trust on the notification. You might see a similar notice in iTunes, asking whether you want to allow the computer to access the device. Click Continue to connect.
Disappearing Device Sidebar
Unlike older versions of iTunes, iTunes 12 doesn't always show your connected devices in a sidebar. If you click and drag a file in your iTunes library, you'll see the sidebar for managing content manually, but it disappears again when you drop the file. To keep the sidebar open, use the Playlists view.
Reinstall iTunes and Update iOS
If nothing else solves your problem, reinstalling iTunes might clear up the issue. Remove iTunes from your computer, and then download a new copy and install it. Also make sure your device has the newest version of iOS: In the Settings app, tap General and Software Update.