Hi all,
As I had the same issue as the original poster I wanted to post up my solution for future readers needs:
1. Format a flash drive using the FAT filesystem
2. Download the JUNOS OS Image and obtain the boot loader files via the procedure listed at https://kb.juniper.net/InfoCenter/index?page=content&id=KB30427
3. Copy the JUNOS image (.tgz file), the uboot, and loader_crc on to the flash drive
4. Insert the flash drive into the USB slot 0 on the SRX and boot up the device
5. When presented with the following U-Boot message press the space bar to enter the U-Boot loader
Press SPACE to abort autoboot in 1 seconds
6. Verify that the files can be seen on the flash drive
=> fatls usb 1:1
.spotlight-v100/
.fseventsd/
297184 loader_crc
4096 ._loader_crc
639364 uboot
4096 ._uboot
4 file(s), 2 dir(s)
7. Copy the uboot and loader_crc files to the SRX using the below sequence of commands (output below)
fatload usb 1:1 0x100000 uboot
bootloader upgrade u-boot active 0x100000
reset
fatload usb 1:1 0x100000 loader_crc
bootloader upgrade loader 0x100000
Reset
8. Output of operation below
=> fatload usb 1:1 0x100000 uboot
reading uboot
639364 bytes read
=> bootloader upgrade u-boot active 0x100000
Checking sanity of backup u-boot...OK
Un-Protected 10 sectors
.......... done
Erased 10 sectors
writing to flash...
Verifying the new u-boot... OK
=> reset
U-Boot 1.1.6-JNPR-2.8 (Build time: Feb 10 2015 - 01:03:41)
Initializing memory this may take some time...
Measured DDR clock 333.28 MHz
SRX_240H2 board revision major:2, minor:11, serial #: ACLT4470
OCTEON CN5230R-SCP pass 2.0, Core clock: 600 MHz, DDR clock: 333 MHz (666 Mhz data rate)
DRAM: 2048 MB
Starting Memory POST...
Checking datalines... OK
Checking address lines... OK
Checking 512K memory for U-Boot... OK.
Running U-Boot CRC Test... OK.
Flash: 4 MB
USB: scanning bus for devices...
Root Hub 0: 4 USB Device(s) found
Root Hub 1: 1 USB Device(s) found
scanning bus for storage devices... 2 Storage Device(s) found
Clearing DRAM........ done
BIST check passed.
PCIe: Initializing port 0
PCIe: Initializing port 1
Boot Media: nand-flash usb
Net: octeth0
Switch driver image not programmed properly in bootflash
Expected 0x3c1c0000, actual 0xaaaaaaaa
POST Passed
Press SPACE to abort autoboot in 1 seconds
## No elf image at address 0x00100000
=> fatload usb 1:1 0x100000 loader_crc
reading loader_crc
297184 bytes read
=> bootloader upgrade loader 0x100000
Un-Protected 16 sectors
................ done
Erased 16 sectors
writing to flash...
Verifying new loader image...OK
=> reset