optimize storage on Mac mini M4

Any folders in your User folder can be moved to an external drive except for Library and Applications.

This includes the Photos Folder if using the Photos App and the Music folder. For Photos though the drive needs to be formatted as APFS, and the Photos app then needs to be pointed to the new location to open the Photo Library by holding down the Option key when opening the Photos app for the first time after moving the folder and Photo Library file.

The Music Library can also be moved to an external drive and again holding down the option key when opening the Music app will let you post it to the new location of the library.

You can create new folders on an external drive, then move your documents and other files to them.

If you are looking to free up space, you can also move your Photos, Music, Movie Libraries to an external hard drive.

Move your Photos library to save space on your Mac - Apple Support

Change where your music files are stored on Mac - Apple Support

Move your iMovie for Mac library - Apple Support

In addition to the previous information, see > Free up storage space on Mac - Apple Support

After you've copied folder with libraries (Photos, Music and Movies) to the external drive before to make sure they work as expected on the EHD before deleting from the internal Drive. If you copy Pictures, Music or Movies folders to the EHD do not delete them from the internal boot drive. Just delete the contents of the folders. Leave the original folders on the boot drive but empty.