https://tinymicros.com/wiki/Resize_E...tion_Under_OSX
I found this, have installed brew, but I understand too little to make it work:
Resize EXT2/EXT3 Partition Under OSX
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e2fsprogs must be installed.
brew update
brew install ext2fuse
brew install Caskroom/cask/osxfuse
brew install e2fsprogs
Insert the SD card
sudo diskutil list
Find the disk number for SD card, typically last the entry
sudo diskutil unmountdisk /dev/diskX
sudo zxcat imagename.xz | dd bs=16m of=/dev/diskX
sudo diskutil unmountdisk /dev/diskX
sudo fdisk -e /dev/diskX
e 2
n
<enter>
<enter>
w
q
sudo /usr/local/Cellar/e2fsprogs/1.42.11/sbin/resize2fs -f /dev/diskXs2
sudo diskutil eject /dev/diskX
--
e2fsprogs is now at v. 1.45.6