Sounds like the board is trying to recover (code 30) from a bad BIOS load/CMOS data. If using the motherboard jumper to clear the CMOS data doesn't help, try unplugging the mains, removing the motherboard battery and letting the board sit for an hour or more.
Based upon several threads dealing with similar problems with other series 7 boards, the best chance of success for the BIOS reload appears to happen when:
The flash drive is formatted with either the FAT16 or FAT 32 file system. The Microsoft exFAT and NTFS file systems cannot be read by the BIOS;
The BIOS image file is the only file on the flash device;
The flash device holding only the BIOS image file is inserted into one of the rear panel USB 2.0 ports controlled by the Intel ICH (black or yellow under the current Intel convention).
Thanks for the answer. That's pretty much what I have tried to do, but I'm going to make another try on the yellow ports, with a smaller capacity flash drive and an older BIOS.
Update : No change with a different flash drive/USB port/RAM (the status LED is stuck on MEM, but I'm guessing that's the onboard memory, and not the DDR3).
DId you manage to fix it? I'm having a similar problem with my DZ77GA-70X, but it stays on post code 20 and the restart...
