OK Nik, thank you for the prompt reply. I have dug further (and it seems to get weirder!) Let me try to be more explicit with the workflow:
1. .ARWs loaded from my Sony camera to my external SSD attached to the Mac (the SSD is encrypted)
2. Use Nitro and only Nitro to do some basic edits on some of the images. XMP files now in tandem with ARWs, everything as it should be.
3. Go away, take iPad and SSD with me
This is where it gets odd. First case, editing files while they are still on the SSD, which is simply plugged to the iPad
1. Everything looks fine; the previous edits from the Mac are all there, and…
2. If I add more edits to an already edited image everything still seems OK …until I change the Flag or Star-rating.
3. If I do that, the app glitches slightly (the editing panel seems to close and possibly open some of the options, e.g. if White Balance is expanded, it minimises it), and then the edits revert to how they were from the Mac and before I did anything on the iPad. This is repeatable.
4. The XMP file carries a Modification time that is consistent with the iPad edit and is later and not the same as the original edits on the Mac
If I edit an image on the iPad stored on the SSD, and which has not been edited before on the Mac, things work as they should. The XMP file is written to the SSD and I can use the flag/star buttons and they don’t revert the edits, i.e no problems
If I copy the files with just Mac edits (ARW & XMP pairs) from the SSD to my iPad and edit them, basically things work. I can add edits and flag/star the image and it is all saved and works.
Now the last twist: If I copy the ARWs and XMPs from the Encrypted SSD to an SSD that is not encrypted, the crazy flag/star things happens again, i.e. edits applied on the iPad disappear when changing the flag/star rating
(The SSDs are both SanDisk, formatted on the Mac)
Thanks in advance for any thoughts, John