Me and my wife have the same issue. She on her 64Gb iPad and me on my iPad Pro with 256GB of storage. It pops up regularly. Rebooting sometime helps, but mostly it doens't. I have just used 38GB of 256GB on my iPad and my wife around 32GB of the 64GB available. We have a 2TB iCloud plan and there we still have 1,4TB left, so enough space there as well.
When we import the pictures via our Mac Mini we have no issue. They upload fine and are available in the Photo app on our iPhones/iPads afterwards without giving some error message.
The above issue only happens while importing through adapters and those are from Apple. This really is a mess and for us it really degrades the fun we have with our devices.
There are however work arounds of which the latter is really astonishing.
- The first one is through wifi. We have cameras that support wifi and by using the app provided by the manufacturer we can copy them to the iPad and see them in the Photos app. No issue, but takes long if you have several hundred of pictures to copy.
- The second is a work around making use of the official Apple adapters. Copy the image to the local storage on your iPad and when done select the copied pictures and add them to your picture library via sharing.
The latter option means that you have the pictures twice on your device without having a space issue. Just to emphasise that space isn't an issue. There really is a bug with the Photo app somehow. And this has been there for a very loooooong time and not yet solved.
And before someone suggests that other devices that are linked to our iCloud accounts are to blame. On our iPhones we have more that 60% of storage available on those devices.
So Apple. What is the underlying issue?