Q: Ipad connects to Macbook Pro, but not to my iMac (it beeps). Any ideas?

Hello duncan55,

Thanks for using Apple Support Communities.

From your post I understand that when your iPhone is plugged into your iMac, it is appearing to be recognized and then unrecognized, along with it not constantly charging.  Given the same device used with the same cable on another computer functions normally, then it appears to be isolated to the iMac.  Have you tried using another USB port on the iMac?  For troubleshooting USB issues, please follow the steps n the link below.

Try these steps to check the connection between your device or computer. You might need to do only one of these steps to resolve the issue, so test your connection after each step.

1. See if the cable connection at the device or computer has come loose or unplugged.
   Unplug the cable you're using to connect your iOS device from the USB port on your computer, and then reconnect it. Then unplug it from your iOS device, and reconnect it.
2. Remove any third-party battery packs or cases from the device and ensure that the USB cable is connected directly to the port on the device.
3. If your Lightning or 30-pin cable is connected to a keyboard, display or USB hub, try plugging it directly into a USB port on the computer instead. 
4. Close any virtualization apps (such as Parallels or VMWare).
   These apps might interfere with your computer's ability to communicate over USB if they aren't properly configured, or up to date. If this resolves the issue, contact the developer of the app to see if an update is available.
5. Restart your computer.
6. Restart your iPhone, iPad, or iPod touch.
7. Check for software updates. You can download the latest version of iTunes from Apple. If you're using a Mac, you can check for updates to OS X and other software using the Mac App Store. 
8. If you're using a non-Apple computer, check for firmware updates for your computer or USB card from the manufacturer's website.
9. If your iOS device displays the Connect to iTunes screen, force your device to restart, and then restore it.
10. If you're using an accessory or third-party cable to connect your device, try using an Apple cable with the device instead.
11. Disconnect other USB devices from your computer, except for your keyboard and mouse. Then, reconnect your iOS device. If the issue is resolved, reconnect the other USB devices one at a time. Make sure your iOS device is still recognized by the computer after adding back each device. 

If one of these steps resolves the issue, but it comes back later, make sure you haven't changed how the device is connected. For example, check to see if you plugged the device into a specific accessory, or used a specific cable that reintroduced the issue.

Take care.

Hi duncan55,

Thank you for using the Apple Support Community. Due to the nature of your issue or question you may find more information by using another one of Apple's support resources - https://getsupport.apple.com/GetproductgroupList.action

Take care.