I had already done that.
It is really weird.
When I import from images from a folder created by Data Rescue ( stored on a different external drive ) the Mac will import some of the NEF images perfectly, yet then reject a whole batch stating '
I have submitted a Feedback Assistant report to Apple.
On that report I gave a specific 'rejected' image because it was 
If select Get Info for file 17548.nef one gets

For some reason that file is 'Locked' ( wasn't by me?)
This app will not recognise the file on Desktop.
I unlocked file by 'duplicating it as a TIFF. ( see snap of two files )
So does this mean I have to convert thousands of images to TIFF's.
And the resultant tiff is 7X size ...a whopping 209.7mb whilst .nef was 28.3.
Guess I'll just go thru the files I really want to keep and convert em to TIFFS for editing in Pixelmator Pro and import them as 'jpegs in the rebuilt Photos?
Thanks for reading.... a lot of images but 'a picture is worth.......

Paul