Re: SRX won't boot and won't start "loader.

Were you able to confirm or reformat the USB drives using fat32 format?

Also instead of TFTP you can try the USB install, for this you don't need to boot to USB only have the install file on the USB instead of the TFTP server.

http://www.juniper.net/techpubs/en_US/junos14.1/topics/task/installation/security-junos-os-boot-loader-usb-storage-device-srx-series-device-installing.html