sometimes I can back up to an older computer and sometimes phone doesn't show up in iTunes, why not?
Why doesn't my phone show up on my older MBP in ITUNES?
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For iOS 14 or later you need iTunes 12.8.2.3 or better running on OS X 10.11.6 or better. See How to upgrade to OS X El Capitan - Apple Support if required.
Assuming a suitable version of iTunes is in place then in Finder use the menu item Go > Go to Folder..., copy the following text, paste it into the dialog box and press enter:
/System/Library/PrivateFrameworks/MobileDevice.framework/Versions/Current/Resources
Unlock your device and connect it to USB. Double click on MobileDeviceUpdater in the folder that has opened. It should notify you of a software update. Install it and iTunes should hopefully recognize your device.
If these steps haven't worked you can also try reinstalling iTunes12.8.2.3.dmg (El Capitan/Sierra) or iTunes12.8.3.1 (High Sierra) while your device is connected and unlocked.
tt2