iTunes couldn't complete the iPad backup because iPad has been disconnected

Did you try quitting everything, rebooting the iPad and the Mac and then starti all over again? If you didn't try that, that's always the first thing that I do when my iPads and iTunes on my Mac don't want to play nice together. Most of the time, that's all that I need to do to resolve the issue. I would also try another cable, if you have one, and/or another USB port on the Mac.

If that doesn't work, you might have a corrupt backup, or a corrupt app or document causing problems. Close all of the apps on the iPad, force restart and then try again.

Close all apps. Press the Home button two times quickly. You'll see small previews of your recently used apps. Swipe left to find the app you want to close. Swipe up on the app's preview to close it.

Now force restart the device. To force restart the device, press and hold both the Sleep/Wake and Home buttons for at least 10 seconds, until you see the Apple logo.

IF that fails, see if you have multiple backups in iTunes. Launch iTunes - no need to connect the iPad to do this - and go to iTunes>Preferences>Devices. You will see all of your iOS backups if you have multiple backups, you might want to delete the oldest one and then try syncing again.

Sorry, I have been busy for a few hours and unfortunately I'm off again for a little while, but it want to keep this going. Have you tried forcing a manual backup? Connect the iPad, select it from the devices icon, then click on backup in the summary pane of iTunes? Can you transfer purchases successfully?

Redownload or transfer your iTunes Store purchases from an iPhone, iPad or iPod to a computer - Apple Support

Can you backup with iCloud or have you ever backed up with iCloud? You can use both. I do. My iTunes backups are my primary backups, but I use iCloud as my secondary backup. iCloud will not help with the iTunes situation, but it will give you another backup.

IF you cannot make a backup with iCloud, that would indicate that something is amiss on the iPad. What puzzles me is that you would get that same message if the backup process fails with iCloud. I don't think I have ever seen that before. So just to confirm, when you tried to use iCloud for a backup, you still got the iPad disconnected message, is that correct? Have you tried toggling WiFi sync in the Summary pane of iTunes when the iPad is connected to the Mac?

This is an old discussion, but much of this still applies. Lawrence Finch has some suggestions that worked for some of the people that got involved here.

iTunes could not backup iPhone because the iPhone disconnected

It doesn't matter if it's an iPhone, iPad or iPod Touch. The same steps would work on any or all three of those devices.

i Don't think this will fox it, but it's not going to hurt anything either. Reset all settings on the iPad. Settings>General>Reset>Reset all settings. You will have to join your prefed WiFi networks again meaning that you will have to enter the network passwords again. If you don't know yours, don't do this until you find the password. You don't need to be unable to connect to your WiFi network on top of your other issues right now.

One of the really puzzling things about this is why you would be getting the iTunes message when you try to backup with iCloud. You aren't trying to backup with iCloud while your iPad is connected to iTunes are you? Forgive me for asking, but I simply cannot fathom getting an error message from iTunes without iTunes somehow being involved.

Well that explains it. you do not need to connect iTunes at all. In fact, don't. Read this.

How to back up your devices using iCloud or iTunes - Apple Support

Tolong saya dah tak tahu dah..dah banyak saya tekan-tekan