Cannot flash new code

[ScruffR]

That might well be the reason.
Downgrading to 0.6.2 from 0.7.0-rc.2 requires youto downgrade the bootloader too - see release notes

Particle Device OS Updates Thread - #48 by BDub

Note about Downgrading [Photon/P1] OTA: First downgrade system-part2 back to 0.6.2. Any attempts to write to the DCT while the device is in DFU mode will fail now. At this point, attempting to downgrade system-part1 will fail silently, since the new bootloader (v100) cannot access DCT and read module update info, thus, in order to downgrade from 0.7.0-rc.2 to 0.6.2, photon/p1 firmware modules should be flashed in the following order with 0.6.2 system binaries: system-part2 -> bootloader (v0.6.2) -> system-part1.