I see you must be running the latest code--that pop-up was put in recently.
It means that Ekos can't save the file that was captured. It tried to write but failed.
Before the popup, Ekos would just hang in the "downloading" phase, not good, but now at least it lets you know.
So, why can't it write the file?
Is it possible that /media/stellarmate doesn't exist, or /media/stellarmate/Picture (e.g. should it be Pictures with an s?).
Is your drive full? It tries to create directories, but KStars often doesn't have permission to create high-level directories.
In a terminal you can make sure the exact directory names exist and the permissions are good with the ls command.
You can also experiment by changing the destination to a directory you know you can write.
Thank you for suggesting some things to check for.
I am seeing that images can be saved to the pi but can't save to the media disk.
Everything else doesn't show any issue in what I see. The folder's seem to be in order as far as not being filled. The destination shows up correct through digging for the Media folder in stellarmate, the Picture folder also shows up properly in the pi's file explored and this worked fine prior to this week.
I likely will try formatting my media disk, to see if there might have been some corruption; otherwise I'm really perplexed.
I will update here if the formatting helps.
If there is any other setting that might change itself to cause this I'd be all ears.
I think the main thing you should check is if you can write a file to that directory, by other means (i.e. not using KStars). For instance, copying some file there. Perhaps it is mounted read-only? That's a thing I've seen over the years, e.g. something odd happened when the system was mounting the disk.
That is often a hardware issue. It can be something as simple as it wasn't plugged in well, and replugging it works.
Pulling it out of the Pi and plugging in to the Mac tests the hardware, I guess, but you really want to do these "can I write" tests on the Pi itself.
Yesterday evening I reached out to Stellarmate support and Jasem provided the details for restoring drive write status.
In my case it would be sudo chown -R stellarmate:stellarmate /media/stellarmate/Picture
I tried this out and it actually restored my ability to take images on my flash drive!
Jasem did say that by default the permissions should allow write on the flash drive. The remaining question in which I might never know is why my drive lost it's write privilege from the Stellarmate OS on the pi device.
Hopefully this will help others in a similar predicament. I really appreciate the help in being able to identify and solve the issue.