In a new support document, Apple has indicated that the first public beta release of macOS Catalina does not support switching between multiple iTunes libraries. This also applies to the first two developer betas of macOS Catalina.
Initial betas of macOS Catalina can only open the previously selected iTunes library on your Mac, according to Apple. Before you install the macOS Catalina beta, choose the iTunes library that you want to use by following these steps:
1. In the iTunes app on your Mac, choose iTunes > Quit iTunes.
2. Hold down the Option key while you open iTunes.
3. In the window that appears, click Choose Library.
Apple split iTunes into three apps on macOS Catalina: Music, TV, and Podcasts.
The iTunes library that you choose will be the library that opens with the beta versions of Apple Music, Apple TV, and Apple Podcasts on macOS Catalina, according to Apple. Your other music files, media, and additional iTunes libraries will remain wherever you have saved them, per the support document.
Apple’s support document suggests that multiple iTunes libraries will be supported in the general release of macOS Catalina this fall.
iTunes has long allowed users to have multiple libraries. Borrowing Apple’s examples, you could have a library of holiday music that would not appear in iTunes the rest of the year, or you could keep your music in a library on your computer and your movies in a library on an external storage device.