Transferring iPhone photos to Mac where the Photos Library is in an external storage

How is your external drive formatted? To avoid damaging the Photos Library an external drive must be formatted in either APFS format or Mac OS Extended (Journaled) format. Additionally, the drive 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

You don't talk about iCloud-- do you use iCloud Photos?

Mitochondrion wrote:

My iPhone and Mac is running out of storage space. I want to remove photographs from my iPhone to make space to take new photographs. I have an external hard disk drive and its storage capacity is larger than my Mac internal storage space. I decided to copy my Mac Photos Library to the external storage and make Photos app open the Photos Library from the external storage instead of the Mac internal storage. I would like to transfer iPhone photographs to Photos where the library is in the external storage. I plugged my iPhone to the Mac and it recognised it and it says "Preparing to import from iPhone" for a long time. The Photos app is not frozen nor is my Mac.

I waited for a while and it still says that. Should I be patient and wait however long it takes or is there another way to accomplish this? I tried rebooting my Mac and it is still the same.

Yup you should wait it out, especially if you have a bunch of photos to import, same thing happens on windows pc.

Mitochondrion wrote: how long did you have to wait?

It seems quick. If you have a large Library, then it's best to have a fast SSD, though.

And you're right, Case Sensitive is not supported. I have some case sensitive drives, because I have old photos with backups, and some of them are JPGs and backups may be jpgs, and I don't want to re-name them all. But Photos doesn't want to have to deal with that.