How can I transfer music from external hard drive to iPhone?

Hi. If iTunes is showing only podcasts and audiobooks then you have installed one or more of the following; Apple Music for Windows, Apple TV for Windows, Apple Devices for Windows. The intention is that you would use Apple Devices to manage the transfer of content to your device, unless you also have the Apple Music subscription service in which case it should all sync seamlessly via the cloud. Remove whichever of the three you have installed so iTunes works as it is supposed to.

If you're starting over with a new iTunes library then I recommend holding shift while starting iTunes so you can create a new library on your external drive, e.g. at D:\iTunes if the external is D:\, then you move/copy the songs into D:\iTunes\iTunes Media before adding that folder to the library to import the content. That way everything is in the one iTunes folder which can be easily backed up and moved to a new drive or computer when needed.

Syncing an iPhone to a new (to it) computer/library will want to erase the current content and replace it with what is in the library. You should take the opportunity to transfer your purchases first. If you have non-purchased media on the device that isn't in your library see Recover your iTunes library from your iPod or iOS device - Apple Community.

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