First you can not downgrade from 8.3 (C-Mode only) to 7-Mode - you need to perform new installation of 7-Mode. Also disk lables in 8.3 are not recognized (or, better, accepted) by 8.2 which explains panic - it does not see any disk. So to install new 8.2 version you would need to
downgrade to 8.2.x C-Mode. Follow standard documentation, if it is new system it should be easy. You do not care about preserving data anyway. This will downgrade disk labels.