When recording ADATs with any MOTU Audio product, the OPTION of ADAT optical clock source must be chosen. The trick is that with the 828, this option does not appear in your clock source until you ENABLE the Optical INPUTS to either TosLink or ADAT...
ENABLING the outputs is not enough...
once you enable those inputs for optical, a 4th choice appears in your clock source box
"ADAT OPTICAL"