A fluke with the intent to justify the use of a U1 over a U3. It is possible but is it sustainable over the life of the card? and why would they sell a U3 card as a U1 (Or V since that is the measure I use)? The U1 is designed for the lower speed and the U3 is designed for a higher speed. Or is it based on factory tests on the production line as to which get segregated for V10, V30 or V90 and sold as such? It would be interesting to find out. But for me; the bottom line is the spec says V30....and with prices so low nowadays.....why accept the risk or justify based on the shooting mode I think I will be in (like 1080 vs 2.7 or 4K)?
I'm not trying to justify anything. I have
1x San disk extreme Micro SDXC V30/U3
2x Lexar Micro-SDXC 633x Class 10 V30/U3 [
A1] (which is on DJI's list of approved cards
1x Philips Micro-SDXC class 10
1x Sandisk ultra Micro-SDXC class 10
2x Sandisk ultra SDXC (not micro) class 10, U1 "80MB/s"
at least 4x Kingston 64GB 80MB/sec Micro SDXC U1 class 10
I tried them with testing tool which comes with windows. Not guaranteed to be representative of what goes on in a drone, but not exactly a fluke.
I bought the U3s specifically for the drone and everything else with my DSLR in mind. I ordered the Lexars because they're on DJIs list but my supplier couldn't deliver before the drone arrived, so I got the San disk from Amazon. It's useful to know if I can just use the cards as they come to hand or if I need to keep anything less than U3 away from the drone - which is similar to the original question.
One might ask how come Kingston's U1 is faster at writes than Lexar's U3 ? Possibly the Kingston comfortably exceeds the U3 requirement on some counts and doesn't meet it on others, and Lexar meets it on all counts without exceeding it by much.
With Sandisk it seems different cards have been optimized for different things, but a given manufacturing process should always produce the same speed (it was said that at one time all intel chips CPU were the fastest speed and those stamped with a slower speed had failed at high speed). It's not entirely the memory speed - the interfaces have some effect too. It also would not surprise me if using a different reader changed the bench mark results or if some adapters from Micro to full SD were a drag on performance. With each marketing name trying to out do the last we're left pondering if "Ultra" is more less than "Extreme".
When a speed is quoted it's almost always read speed. Great for knowing if your phone can play 4K movies but not so good for knowing if your drone can record them.
With all that said... the mini can't shoot 4K. it's 2720 x 1530 (as near as makes no difference it's double the pixels of 1920x1080 and half the pixels of 4k) so if a U1 card can handle 4K it can definitely cope with the mini's 2.7K.
The rule of thumb seems to be 4K 30 fps needs 10 MBytes / sec (80Mbit/sec) sustained write-speed , which is class 10 / U1. 2.7K 5MBytes/sec , and 1920x1080 2.5MBytes/sec. I must see what the slowest car I have is and what the mini does with it