The image has to be written verbatim to the USB flash drive for it to be bootable on UEFI and BIOS systems. Here are the recommended tools to do that.

Etcher is a popular and user-friendly tool. It works on Linux, Windows and macOS. Download it from balena.io, start the program, select the downloaded NixOS ISO, then select the USB flash drive and flash it. Etcher reports errors and usage statistics by default, which can be disabled in the settings. An alternative is USBImager, which is very simple and does not connect to the internet. Download the version with write-only (wo) interface for your system. Start the program, select the image, select the USB flash drive and click Write.

Plug in the USB flash drive. Find the corresponding device with lsblk. You can distinguish them by their size. Make sure all partitions on the device are properly unmounted. Replace sdX with your device (e.g. sdb). sudo umount /dev/sdX* Then use the dd utility to write the image to the USB flash drive. sudo dd if=<path-to-image> of=/dev/sdX bs=4M conv=fsync

Plug in the USB flash drive. Find the corresponding device with diskutil list. You can distinguish them by their size. Make sure all partitions on the device are properly unmounted. Replace diskX with your device (e.g. disk1). diskutil unmountDisk diskX Then use the dd utility to write the image to the USB flash drive. sudo dd if=<path-to-image> of=/dev/rdiskX bs=4m After dd completes, a GUI dialog "The disk you inserted was not readable by this computer" will pop up, which can be ignored. Using the 'raw' rdiskX device instead of diskX with dd completes in minutes instead of hours. Eject the disk when it is finished. diskutil eject /dev/diskX