Apple Music not loading all songs on new mac/ "Transfer" error

Hi there kevinpnb,

Thanks for joining us in Apple Support Communities! We understand you're seeing an alert on your new MacBook Air indicating the device is already associated with another Apple ID when trying to play specific songs in Apple Music. From what we can tell, this happened after importing the songs from your music library stored on an external storage device. We're glad you're here, and we can provide some guidance.

"You can have ten devices (no more than five computers) associated with your Apple ID for purchases at one time.

When you try to associate another device that was previously associated, you might see a message that says, "This device is associated with another Apple ID. You cannot associate this device with another Apple ID for [number] days." Depending on when you associated your device with your Apple ID, you might need to wait up to 90 days to associate the device with another Apple ID."

Have a look at this link to learn more:

View and remove your associated devices for purchases

Next, try the suggestions in this link to see if they help:

If you have problems playing music on Mac

We hope this information is helpful. Don’t hesitate to let us know if you have any more questions.

Cheers.

See Authorization vs. association - Apple Community for further background on this issue. Have you tried to download past purchases from more than one account? Your iCloud Music Library can only contain content from one Apple ID.

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