How long does it take to transfer photos from MacBook Pro to iPhone?

That's helpful information.

How much free storage do you have on your iPhone? on your Mac? Do you have at least 10% free?

What do you mean by "synchronise?" Do you mean that you are using iCloud Photos? Assuming so:

• What does Photos say at the bottom? Mine says "Synced with iCloud Just Now"

• What do you see at the AppleID icon at the top right of the Library view on your iPhone?

• Do you have Optimize Storage turned on for Photos?
• What do you see at iCloud.com?
• How much free storage do you have on your iPhone?

on your Mac?

at iCloud?

• Has your iPhone been left plugged in, locked, with stable internet, and unused?

This information should help us figure it out, so let us know…

48 hours is a long time. I use iCloud in part because cable transfer can be flaky. Have you tried a forced restart or your phone? 

Force restart iPhone - Apple Support

This is different from a simple Power Off. I had to continue holding the button down past the "slide to power off" message, and then I released the button only after  the Apple Logo appeared indicating a re-start. 

On the Mac, have you re-booted? You should also try a re-start in Safe Mode. This bypasses certain potentially disruptive processes, and it often helps. Safe Mode is different for different computers, so see this: 

           Start up your Mac in safe mode - Apple Support 

"iPod Photo Cache"

In my setup that folder is always created after exporting Mac's Photos library (or a Finder folder) via USB to iPad.

Is that screenshot of the Mac Photos library's contents? Have you ever successfully exported to iOS/iPadOS device? Where is your Photos library stored? On the internal volume or on an external APFS or MacOS Extended disk?

I have occasionally deleted "iPod Photo Cache" folder when the sync has otherwise repeatedly failed. I usually sync an empty folder before doing that so the iPad has no existing photos that might intervene (sometimes that times out if there are tens of thousands images on the iPad, so I repeat the sync until there are no images on the iPad). But I take care not to otherwise mess with the Photos library contents because it might corrupt the library. I suggest backing up the library before doing this, just in case.

vogelsgesang wrote: …have both rebooted …

… in Safe mode? What else have you done? Have you tried making a new Library and transferring to it? Have you tried doing this with a new user? Have you tried rebuilding the Library? Have you used the app Image Capture?

Syncing from the Mac Photos library to iOS/iPadOS devices via USB has been historically flaky and sometimes very slow.

If all else fails my solution to that has been to backup the library, then open its package and delete the "iPod Photo Cache" folder:

On the Mac quit Photos and backup the Photos library (just in case because messing inside Photos library is generally not recommended because it might corrupt it).

Right click or control-click the "Photos Library.photoslibrary", choose "Show Package Contents" and delete the "iPod Photo Cache" folder (it holds a database of previous syncs and might have been corrupted. It is re-created in the next step). Empty trash and restart the computer.

Then try again.

vogelsgesang wrote:

this time I found the "Ipod Photo cache" and deleted it. The process of coping the pictures is still continuing, after 24h of running.

I guess you found "iPod Photo Cache" before my today's post (why didn't you find it initially??) because your sync has been running 24+ hours, right? ...I'd have cancelled export before 24 hours with my 400GB, ≈40 000 files library (how large is your Photos library??). Did you nuke the previous iPhone library before re-trying as I described in my previous post? I do not have solid proof but nowadays I reboot, open that system Photos library and then sync it to iPad via Finder and USB (...oh, and I also rotate chicken clockwise on top of the devices ;-] -- just kidding).

And just to make sure I ask again (somehow in these forums only one question to the OP is answered...): Where is your Photos library stored? On the internal volume or on an external APFS or MacOS Extended disk?

vogelsgesang wrote:

Iphone is connected to the mac by UBS and the syn does not show any change. It starts and then after step 3 it just finishes without adding any pictures although I still have 73.000 on my mac( under the picture file), but only some 1800 on the Iphone.

Did you try to empty the iPhone from old synced images by syncing an empty folder from the Mac to the iPhone until there are no old synced images (you might have to do that several times if it times out before finishing)? You could leave only images that are on the camera roll although I usually empty also it by copying the images to the Mac to more clearly see that the iPhone has zero images. Then delete that "iPod Photo Cache". Then open the Photos library and set it as a system library. Then reboot and open Photos and leave it to the background. Then try to sync from the Finder to the iPhone and see if it then works.

Does it work if you create a new dummy Photos library with, say, 1000 newish regular mint images and movies and sync that? If it works, it might hint that the old library has some rogue corrupted image that halts the sync process. It might be very difficult to find out which image it is, though.

If it then does not work, I am out of ideas except doing a clean install on both the Mac and iPhone but I have never needed to do that. Or try to sync from a dummy new user account to rule out some preference glitch in your account.