What's your go to SD card on for your Mini?  I was flying the other day and put in a 32gb Samsung EVO Select U1, and took a couple of videos.  When I brought the bird home, I tried downloading them (and later anything) and it would always error out.  I got home, and put the SD card in my computer and copied all the files.  I put the card back in the Mini, and it would work on seeing the card for a bit and then tell me 'no card deteted'. So I formated the card (Fat32) on my computer and put it in the Mini and it saw it briefly, but when I asked the Mini to format the card it started and after 30 seconds, came back with 'no card detected'.  I formated the sd card again on the computer and never had the Mini see it again.  I tried another of the same card, and got the same results.  Then I pulled the 64gb Samsung EVO Select U3 from my Mavic 2 bag, and it worked fine.  

Is this expected behaviour, is the U1 card on the edge of not being fast enough?  

The 32gb card is 'old' and doesn't have a UHS rating but since it has a U rating it must be UHS1, I think the UHS rating is not as important as the U rating, the former refering to bus speed and the latter the speed class.  I feel like I'd been using U1 cards for a while with no troubles with the Mini (and a Phantom 3), but maybe bought the 64gb U3 cards because they didn't work in the M2 (the memory is foggy).

I'm about to order a couple of spare 32gb cards off Amazon (Samsung Extreme UHS3/U3).  These should work. What cards are you guys using.  I'm going to hold my order until I get feedback from smarter people than me.

Warning! This is not feedback from smarter person but Samsung EVO PLUS micro SDXC class 10 UHS-3 works great (but short time experience).
EDIT: Only Samsung Pro Endurance and Evo Plus are on Supported cards list...

Hi. I am sorry for the trouble this has caused. With regards to the SD card. It is suggested to use a UHS-I Speed Class 3 SD card because of the write speed. Since the DJI Mavic Mini's minimum resolution is FHD
( Full High Definition ). Please refer to the image I will be posting for the suggested SD card for the DJI Mavic Mini. Thank you.

Do not use U1 card on anything that goes above 1080@30.

Sandisk Extreme pro 64gb

Well if you'd just read the manual...

Mini records at 40Mbit/s and you were trying to use max 20Mbit/s write speed capable card, see the dilemma?

Just pick one from the official supported card list and you're golden. Samsung Evo Plus 64Gb (my go-to card) costs like 10bucks nowadays so it shouldn't break the bank either.

Thanks everyone - I appreciate the help.

Got Samsung Evo 64 GB UHS-I U3 , approved by DJI. Very happy with it.

Sandisk Extreme but make sure you buy it somewhere really reputable there are lots of fakes or relabelled cards out there.

Use a sd card V30 or faster, only buy well known brands from safe stores. Then you're sure not to end up with fakes

Miixxa Posted at 1-28 09:35
Well if you'd just read the manual...

Mini records at 40Mbit/s and you were trying to use max 20Mbit/s write speed capable card, see the dilemma?

This is not correct. Mini records at 40mbps, card is rated at 20MBps, which is technically 160mbps.

1 MBps = 8mbps
Why is OP formatting the card in Fat32 format? That's ancient, and has a file size limitation of 4GB

kokoraki Posted at 1-29 03:05
This is not correct. Mini records at 40mbps, card is rated at 20MBps, which is technically 160mbps.

1 MBps = 8mbps

So, exfat?

Porktree Posted at 1-29 08:25
So, exfat?

Yup, think exFat replaced Fat32 as the default when larger/faster cards came out, could be wrong though. This might be why the card is glitchy and not recognised by the MM

We had similar experience with older SD cards.  Record time with them was limited to a very short time.  Upgraded to faster and it records long durations no problem.  Get the fastest SD you can find.

I was using a SanDisk Ultra 64GB. Just replaced it with a SanDisk Extreme Pro 64GB. Have not tried it yet.

kokoraki Posted at 1-29 03:05
This is not correct. Mini records at 40mbps, card is rated at 20MBps, which is technically 160mbps.

1 MBps = 8mbps

"Why is OP formatting the card in Fat32 format? That's ancient, and has a file size limitation of 4GB"

You can externally format the cards in FAT32 or ExFAT, but it makes no difference at all. The aircraft will split files once they reach 4GB in size, no matter what format you use.