I prepared flash-drive contained:
- \ALTBOOT\
FORCED_UPGRADE
NO_DATA_BACKUP
DISABLE_DATA_BACKUP
NO_CLONE
DISABLE_CLONE
WorkCentre_7830-35-system-sw#07201000409100#.DLM
switched WC off, inserted the drive, and switched on. Don't know that files-markers are out of place, but I did it - got the device entirely wiped.
Thank you all.