Safepoint-how does it work-update manual vrs automatic

I am trying to figure out exactly how safepoint is supposed to function.

I notice that most people use safepoint to an attached USB drive but my setup is to have safepoint write to a old Terastation HS-DHTGL. I had the terastation sitting around and when I started to try safepoint it found the terastation and I decided to use it, since the terastation has raid 1 capabilities. I have two 2.0 TB WD drives installed. These are not Red so I only turn on the terastation to do safepoints.
The initial safepoint (1.1 TB) was created in early March. I realized it would take a lot of time so I just let it run, after about 24 hours all was created and the terastation drives were synced.
I turned off the terastation and just used the MyCloud.
Today, I decided was a good day to do an update. I fired up the terastation and chose the manual update. No problem connecting,
Since I had only made minor changes (about 2 GB) in data to MyCloud, I thought it wouldn’t take long.
Several hours later, the progress reports “Currently Copying” and is at 53 GB of 1.3 TB (4%). I can access the tereastation so I know it isn’t duplicating files and it is adding the new data to the terastation.
I am trying to understand the functioning of safepoint.
Is it just manually accessing every file and comparing that file to what is on the backup?
Does MyCloud have an index of changes?
Would an automatic update behave in the same manner?

Being new to safepoint, I am striving to understand how it works so I can ensure that I don’t lose data.

Only one file Safepoint should be kept unless another one is manually created. Otherwise any file update would trigger a full copy and fill out the hard drive extremely fast.

Which firmware revision do you use? Has this behavior been presented before?

MyCloud is version v04.04.02-105

I just did an update of the original safepoint.