Using sz

brew install lrzsz

Get fortinet into ymodem mode:

resetting
FortiGate-60F (00:0Ver:00000000
Serial number: FGT000000000
CPU: 1200MHz
Total RAM: 2 GBboot device...
Initializing MAC... nplite#0
Please wait for OS to boot, or press any configuration

It will erase data in boot device. Continue? [yes/no]:o

[C]: Configure TFTP parameters.
[R]: Review TFTP: Initiate TFTP firmware transfer.
[F]: Format boot device.
[I]: System information.
[B]: Boot with band set as default.
[Q]: Quit me to boot.
[H]list of options.

Enter s for serial update

Quickly replug the serial console usb

And make sure the serial console wont be busy. The fortinet listen to binary update may be auto timeout and exit at about 60 sec.

Enter command below:

sz -b -vv  --ymodem  ~/Downloads/firmware.out  > /dev/cu.usbserial < /dev/cu.usbserial