My iPad repeats connecting and disconnecting to my iMac with USB

Hi,

There are a couple of similar topics which aren't solved yet out there:

• my iphone 4s connects and disconnects (repeatedly) when connected to my computer
• my iphone 4 connects and disconnects repeatedly when connect to my laptop, how do I fix this?

The problem sometimes happens when my iPad connected to my iMac (it doesn't always happen).

My USB cable is not bad because it can be correctly connected to my another MacBook Pro.

I don't have any idea why it's happening.

But there's an interesting point that the problem doesn't happen while my iPhone 5S is connecting to my iMac.

And it happens again after disconnected my iPhone from the computer.

In addition, it also stopped happening after connected to my MacBook Pro, even without connecting iPhone to my iMac.

It's possibly a hint to solve it.

Any idea?

My setup:

iMac: OS X Yosemite 10.10.2

MacBook Pro: OS X Yosemite 10.10.2

iPad 2: iOS 8.2

iPhone 5S: iOS 8.2