Amazon.com - Kingston 8 GB microSDHC Class 4 Flash Memory Card SDC4/8GB (Standard Packaging)
I have been using these cards since January 9, 2019 and the 8 GB “Frustration-Free Packaging” version linked below since January 23, 2019 (shipped and sold by Amazon). I use these cards directly out of the package with the MP Mini Delta and the MP Select Mini V2 for printing without any formatting prior to use. I also have updated the motion controller firmware many times with these cards and the cards linked below without needing to format the card first. As of July 26, 2019 I have purchased a combination of 12 of these cards and the Frustration-Free Packaging version. As of December 29, 2019 I have not had any problems using the 8 or 9 of these cards that I have put into service.

Amazon.com - Kingston Canvas Select 16GB microSDHC Class 10 microSD Memory Card UHS-I 80MB/s R Flash Memory Card with Adapter (SDCS/16GB)
I ordered this card March 10, 2019. shipped and sold by Amazon. I have successfully used this card directly out of the package with the MP Select Mini V2 and MP Mini Delta for printing with no formatting required. I have not tested if this card will work for updating the motion controller firmware.

Amazon.com - Kingston 8 GB microSDHC Class 4 Flash Memory Card SDC4/8GBET (Frustration-Free Packaging)
This card seems to be either out of stock or maybe no longer available.
I have been using these cards since January 23, 2019 and the 8 GB “Standard Packaging” version linked above since January 9, 2019 (shipped and sold by Amazon). I use these cards directly out of the package with the MP Mini Delta and the MP Select Mini V2 for printing without any formatting prior to use. I also have updated the motion controller firmware many times with these cards and the cards linked above without needing to format the card first. As of July 26, 2019 I have purchased a combination of 12 of these cards and the Standard Packaging version. As of December 29, 2019 I have not had any problems using the 8 or 9 cards that I have put into service.

Amazon.com - Kingston 2 GB microSD Flash Memory Card SDC/2GBSP (Single Pack)
This card seems to be either out of stock or overpriced more than it used to be. I guess I need to identify another known working card that is readily available.
I ordered this card December 14, 2017 from Amazon seller “i77e” to test after a number of people said they have used it without any issues.
I have successfully used this card directly out of the package with the MP Select Mini V2 and the MP Mini Delta for printing and also updating the Motion Controller firmware with no formatting required.

Try any card up to 32GB (yes some 32GB cards will work with the MP Select Mini V2 and the MP Mini Delta). FAT16 and FAT32 both work. Also exFAT works for some. The printer is picky about which cards it will read so if you have multiple microSD cards try them all.

You may also want to try formatting the larger cards such as 16GB and 32GB as FAT32 with different allocation unit/block sizes.

For example:

• FAT32 with a block size of 4096
• FAT32 with a block size of 8192
• FAT32 with a block size of 16384
• FAT32 with a block size of 32768
• FAT32 with a block size of 65536

You can try the same with the lower capacity cards also but using smaller allocation unit/ block sizes. Also try FAT(16) with different allocation unit/ block sizes.

• the printer heats up and never starts printing
• the printer heats up and never starts printing but the progress bar continues to fill
• the printer stops printing but the temps remain and the progress bar continues to fill
• the printer goes through the starting script and then goes where it shouldn't (this could happen for other reasons also such as incorrect slicer settings)

Follow the link below for more information.
Printer Stops or Never Starts Printing