Downloading iPhone pics to Photo App

More questions:

How are you trying to "import pictures from my iPhone?" Are these files that are in a folder on your Mac? Do you plug in your phone and use the app Image Capture? Do you see the phone in the sidebar of Photos? Are you using iCloud to synchronize your pictures? There are so many ways to do things that it's hard to answer without details.

You didn't mention the type of phone, the type of Mac, or what OS each is using. Those are often important to know.

You say your Photos Library" is located in an external drive." How is that drive formatted? To avoid damaging the Photos Library an external drive must be formatted in either APFS format or Mac OS Extended (Journaled) format. The drive must be connected directly to the Mac by cable, not networked, clouded, NASed, etc. Additionally, the drive volume can not have had Time Machine on it since it was formatted. There have been so many problems with using incompatible drives that the newest macOSs won't even allow a Library on a non-Mac formatted drive to open, since there is a chance of damaging the Photos database. See this:

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

If this drive is in a an incompatible format, stop running Photos with it immediately!  A Photos Library can sit on an incompatible drive, but running it may corrupt the database.

Let us know some details…

Define, "doesn't work".

What happens or does not happen exactly?

Does the Photos app not see the iPhone?

Does it see the iPhone, but does nothing when attempting to import photos?

Does it issue an error?

Provide details and someone may be able to advise.