Hello,
Is the amount of music data on the external drives more than your Mac's available storage? If so, the import settings could be the cause.
Change where your iTunes files are stored on Mac - Apple Support
"Add files to your library without adding them to your iTunes folder
When you add an item already on your computer to your iTunes library, iTunes places a copy of the file in your iTunes folder. The original file remains in its current location.
You can change this setting so that files are added to your iTunes library without being added to the iTunes folder. You might want to do this, for example, if you prefer to store video files on an external disk but still want to view them in iTunes.
- In the iTunes app on your Mac, choose iTunes > Preferences, then click Advanced.
- Deselect the checkbox next to “Copy files to iTunes Media folder when adding to library.”
From now on, when you drag a file to your library or choose File > Add to Library, the item appears in iTunes, but the actual file isn’t copied or moved.
Tip: You can temporarily reverse this setting by holding down the Option key while you drag files to the iTunes window."
I hope this helps.