lichenology wrote: …I am not using iCloud. Do I have to?
No, but using iCloud makes synchronizing Photos way easier. And it looks like Apple is paying much more attention to iCloud than to transferring by cable. Also, one reason I asked is that cable transfers and iCloud syncing are largely incompatible.
• What iOS does your iPhone use? just updated to iOS26.3 and still not loading new photos
• What macOS does your Mac use? 12.7.6 Monterey (recently updated - this could be when this became a problem because I have never experienced it before)
Coud be-- it's a big jump!
• Where does your Mac's Photos Library reside-- in the Pictures folder of your internal drive, or on an external drive? or on a network? on iCloud drive? on an external drive
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. The drive must be connected directly to the Mac by cable, not networked, clouded, NASed, etc. 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:
https://support.apple.com/en-us/108345
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.
To be sure, you can click on the Drive icon and use the ⌘-i Info Window

Thee's also an "Ignore Permissions" that should be checked. It normally is.
• Is your Photos Library on your Mac designated as your System Library? You can check at
"Photos' Settings>General" not sure on this - i have general pulled up and library location shows the external hard drive on which my library resides
Sounds right. "Use as System Library" should be grayed out if it's the System Library. Macs, unlike iOS, can have many Libraries, but the System Library is the one that works with cable transfers and Safari and other apps.
• What is your storage on this iPhone? At Settings>General>iPhone Storage, my phone says 50.5GB of 128GB used. What does yours say? Do you have more than 10% free storage on your iPhone? 56.34 of 64GB
Enough, but barely--88%. If you get less than 10% free, things can't get really wonky.
• What is your storage on this Mac? At System Settings>General>iStorage, what does yours say? Do you have more than 10% free storage on your Mac? 54 of 121GB
So you're using that external drive for nearly everything, apparently!
Did you restart in Safe Mode on the Mac? How'd it go?
Did you do a forced restart on the iPhone?
Make sure that the phone is locked when you pug it in-- the could computer should ask you to unlock it. I've heard that if you start with it unlocked, it misses a step.