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

I assume the SRX you pulled the USB snapshot from is the same model as the one that has difficulties.  These instructions suggest a slightly different snapshot command you could try.

https://kb.juniper.net/InfoCenter/index?page=content&id=KB29811

Another approach would be to run a fresh install from the boot loader using the USB as the source drive instead of the TFTP one.

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