For some stupid reason HP didnt put in enough of flash to make you able to keep two firmwares at the same time (making the backup firmware function virtually useless).
So what you need to do when you are about to upgrade a firmware is to first remove the current firmware in flash:
delete /unreserved flash:/<filename>
Dont forget the /unreserved otherwise the file isnt really deleted (and still taking up space in flash).
Then you upload the new firmware to the device (dont forget any bootrom update aswell) and first issue the bootrom update and then the firmware (bootloader) update, after each reboot make sure to save the running config (by just typing "save" and press y to overwrite current saved config).
Once you are done with the rebooting you can verify that everything is up and running along with "display version".
Finally take a backup of your saved config (to store elsewhere).
For more info http://blog.looppacket.net/2012/08/upgrade-firmware-and-bootrom-on-hp-a5120/