Thank you and that makes sense now.
I created a partition only for my astrophotography images. That partition is the only thing I want connected to the raid so it will be backed up constantly when the external SSD is connected, and look exactly like my partition, not the separate images in time machine. Time machine is used for the rest of the hard drive and I am fine with how that works and looks as it is only for an emergency.
I was thinking of 1 external drive and the partition on the internal drive in my MBA. But the partitioned drive does not show up, only the entire internal drive.
So you must select at least two external drives for the raid, but can only use one. But is it possible to use only the partition? Then, as soon as I connect the SSD, it will begin to copy everything from the partition, and look just like the partition. That will be a perfect solution.
I want to do this as I use Time Machine but wish it looked like the drive it was backing up (just 1 current snapshot), instead of the way it does with all those separate backups. Is there another way to get the backed up/copied view that I want? Totally erasing the external SSD then re-copying everything from the partition to it takes too much time as I am over 150 GB and increasing rapidly. As it grows, 80% will not change but what changes needs to stay in the folders like on the partition so I can find it if needed. The reason for all this is I will be eventually moving to a Mac Mini with massive RAM but only 256G storage as I have many 1 & 2 TB fast external SSDs for storage and backup.
