Ok, let's talk about memory cards.
(Jump to the end if you just want the final answer.)
Background
First, the names. SD means "Secure Digital" and is just the type of card. Most computers and card readers support the SD card. Smartphones needed something smaller so they created the "microSD" card. This is much smaller than the original SD, and the microSD is often sold with an adapter so that you can plug it into the bigger SD slot of your computer.
SD cards are limited to 2GB, so they developed SDHC (Secure Digital High Capacity) which can support up to 32GB. To support more than 32GB they developed SDXC (Secure Digital eXtended Capacity) which can support up to 2TB (which should handle most people's needs for quite a while).
When you are shooting in 4K on a Mavic you generate about 1GB of data for every two minutes of video, so a 64GB card will hold around 2 hours of 4K video and require a microSDXC card. 64GB is what I use as I don't shoot more than around 1 hour of video on any given day, after which I move the files onto my computer (which has nearly 30TB of storage) so that the card is ready for the next shoot. If I needed to shoot more than two hours of video, I could either buy another 64GB card, and/or buy a bigger card (128GB or 256GB).
You need to select a reliable card that has a
sustained write speed that will support the M2's maximum video bitrate of 100 Mbps (Mega
bits per second). That means that the card must support a sustained write speed of 100/8 = 12.5 MB/s (Mega
bytes per second).(1 byte = 8 bits). Note that video rates are specified in Mega
bits per second and cards are rated in Mega
bytes per second. Many people do not realize that "b" signifies bits and "B" signifies bytes. A byte is just a chunk of 8 bits.
When you read the marketing specs for a card you will usually find a prominent statement of "supports
up to xxx read and yyy write speeds." Totally useless information as this is how fast it can burst data for a short period of time. Recording video requires the ability to write data at a sustained rate. If it can't keep up you will get dropped frames.
UHS-I (capital "i") and UHS-II indicate the type of bus on the card and do not indicate a speed. UHS-II cards can support higher speeds (312 MB/s) than UHS-I, but few if any consumer devices support that bus. UHS-I cards can support speeds up to 104 MB/s (832 Mbps) which is fast enough for pretty much all current consumer needs so this is what you find for sale. But UHS-I and UHS-II do not tell you what the actual speed of the card is. For this the SD Association has defined speed ratings that indicate the minimum write speed that the card can continuously sustain, which is what you need to know for video applications.
There are a couple of different speed ratings that you should look for when you buy a card, which now often show both. One is the UHS-1 / UHS-3 speed grade rating. These are indicated on the card by a capital "U" with a small "1" or "3" inside the U. UHS-1 (Speed Grade 1) supports a minimum continuous write speed of 10 MB/s, which is not fast enough for the M2 which needs 12.5 MB/s (100 Mbps). UHS-3 (Speed Grade 3) supports a minimum continuous write speed of 30 MB/s (30 X 8 = 240 Mbps) which easily handles the M2's 100 Mbps video bitrate.
The other speed rating for sustained write speed is a capital "V" followed by the MB/s, for example V30 = 30 MB/s. You start to see this rating more often now, especially as the sustained write speeds become faster than the UHS-3 30 MB/s.
The answer to your question
So, with all that as explanation, here's the answer to your question for the "best" microSD card for your M2.
First, from page 38 of the M2 user manual:
"A UHS-I Speed Grade 3 rating microSD card is required due to the fast read and write speeds necessary for high-resolution video data."
My advice:
- Buy a brand name microSDXC card that has a good reliability reputation (I tend to use SanDisk but there are other brands equally good)
- Make sure it is Speed Grade 3 (a U with a 3 inside the U)
- Buy a 64GB card (more than one or bigger if you have long video shoots)
- Buy only what you need. You can spend more for faster cards, but your Mavic will never use the extra speed. For example, the SanDisk Extreme 64GB Speed Grade 3 V30 card is selling today on Amazon for $26.95. The SanDisk Extreme PRO is also Speed Grade 3 V30 but sells for $34.01. What does the PRO give your for the higher price? A faster burst write speed, which is not important for video. No need to buy the PRO, the Extreme is just fine for our M2 video requirements.