Can I install the music library on an external drive with logic 10.5 or do I have to move it afterwards?

From memory the Relocate Sound Library command was introduced in Logic Pro X 10.4.2.

I'm not sure what you mean by "install". On macOS, "installing" an application is usually nothing more than downloading and copying it in the Applications folder. This is the case with Logic. The Sound Library is downloaded and installed from within Logic. Except for the Essential Content, the Library can be relocated before downloading the entire Library.

WD

I'm not sure I understand the question? Afterwards what?

There is a Relocate Sound Library command in Logic. It can be used anytime after Logic is installed.

WD