It definitely looks like you need a flash card if you loose the files in bootflash:
See excerpt from doc:
As the Recover a Supervisor Engine 720 from a Missing/Corrupted Cisco IOS Image or ROMmon Mode section mentions, the Supervisor Engine 720 does not support the Xmodem recovery procedure.
Therefore, always keep a copy of the image in disk0 or disk1, or the MSFC boot Flash. If you do not have any copies on either disk0 or disk1 and you encounter the problem of a missing/corrupted image, the only way to recover is to copy the image on the Flash disk from another switch.