Fix hackrf_open() failed: HackRF not found (-5)

pi@raspberrypi:~ $ hackrf_spiflash -w ~/Downloads/hackrf-2021.03.1/firmware-bin/hackrf_one_usb.bin
File size 35444 bytes.
hackrf_open() failed: HackRF not found (-5)

Solution

Use sudo before the command:

pi@raspberrypi:~ $ sudo hackrf_spiflash -w ~/Downloads/hackrf-2021.03.1/firmware-bin/hackrf_one_usb.bin
File size 35444 bytes.
Erasing SPI flash.
Writing 35444 bytes at 0x000000.