Syncing photos to iPhone 15 Pro Max: 'Remove and Sync' query

Pictures that are transferred through a cable from a computer, perhaps with iTunes, can be shown by Photos, but they're not "owned" by Photos. So these pictures can't be edited or deleted by Photos, and they can't be synchronized by iCloud Photos, for instance. The idea is that they are kept on the computer and transferred to the phone temporarily. That's why older pictures from another computer can't be kept. If you click Remove, those pictures will be lost.

You can instead save these pictures on your phone to Files for later use. Or, you can plug in an external drive to the USBc port on the phone, and then save the older pictures to that drive using the Files app. Then you can import these pictures back to Photos later, and they will then be fully under the control of the Photos app.

Glad it helped…