I learned how to format cards on a Mac from a post on this forum: https://www.openacousticdevices.info/support/sd-card-support/sd-memory-capacity/p-1/dl-5bb715b6dacc230014a1da97 The computer I use to do card formatting has been kept up-to-date, so now has MacOS Mojave 10.14.5. I had success using SanDisk Extreme 128GB cards since last fall, but in the past couple of months, after getting more cards to finally deploy additional AudioMoths I had not yet used, cards were turning up empty, despite use of parameters that had been working. Only a few days ago did a culprit emerge -- use of Disk Utility was not formatting to FAT32, despite choosing "MS-DOS (FAT32)" (also tried ExFAT). Wouldn't have seen the format of the cards without running "diskutil list" on the command line in a terminal window, which shows format details for all your hard drives and attached devices. This showed the failing-to-record cards as: ... /dev/disk9 (external, physical): #: TYPE NAME SIZE IDENTIFIER 0: GUID_partition_scheme *127.9 GB disk9 1: EFI EFI 209.7 MB disk9s1 2: Microsoft Basic Data AUDIOMOTH05 127.7 GB disk9s2 I have quite a few hard drives, so on my system the Lexar card reader with the microsd card is device 9, disk9. The card in this case is named AUDIOMOTH05. Those format details are different from these for cards that were recording fine: ... /dev/disk9 (external, physical): #: TYPE NAME SIZE IDENTIFIER 0: FDisk_partition_scheme *127.9 GB disk9 1: DOS_FAT_32 AUDIOMOTH16 127.8 GB disk9s1 Differences: FDisk_partition_scheme vs. GUID_partition_scheme DOS_FAT_32 vs. "Microsoft Basic Data" I haven't determined how some of my cards got the first (wrong) format, but use of Disk Utility was not changing it to DOS_FAT_32. Testing showed that AudioMoths with the failing-to-record cards would blink red and green LEDs once at the proper recording time, but would not record anything, and would immediately go back to blinking the green LED, waiting for the next recording time. Searching for info, this web page provided another way to format cards on Mac: https://www.michaelcrump.net/the-magical-command-to-get-sdcard-formatted-for-fat32/ Running the following on the command line produced the second format above (FDisk_partition_scheme and DOS_FAT_32) that results in successful audio recordings: diskutil eraseDisk FAT32 AUDIOMOTH05 MBRFormat disk9 FAT32 is the format, AUDIOMOTH05 is the name of the card, MBRFormat means "Master Boot Record" (not the GUID scheme way of formatting), and disk9 is the number of the device holding the card on my system. The disk9 identifier is important to avoid formatting the wrong drive, so "diskutil list" needs to be run first to get that. I have an older Mac with an earlier OS version, but have already reformatted all my cards, so I can't see if an earlier Disk Utility version behaves as above. Regardless, the command line way of formatting is much faster. More details and cautions about dangers of using the command line are given here: http://osxdaily.com/2016/08/30/erase-disk-command-line-mac/ I tried using the formatter from: https://www.sdcard.org/downloads/formatter/index.html but it has not been updated since MacOS High Sierra and did not work on my system, which is running MacOS Mojave. If this program gets an update, it has the advantage of only presenting devices with cards for formatting (hard drives aren't listed). For me, the command line is the way to go now.