I was going to upload terrain data for a large area, so I would not need to think about getting the right files eveytime, but I have a second thougth about it.
Does number of terrain data files on SD card affect FC performance in terms of MCU load?
Are there any side effects except having space used on SD card?
I’ve accidentally loaded some very large areas before. 500+km radius plus a couple of smaller 100km radius areas. I didn’t notice any issues.
On the topic did you see this: Serious terrain data error and how to fix your vehicle
I saw that. This is exactly the reason why I am re-doing terrain data.
But… uploading it over mavftp in MP is… very slow and unreliable, so I just want to upload files directly into SD card, and have this done once till next error disovered 
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That is good news. I will just upload all areas that I am interested in and will not have to think about it for a long time. 