Compare Columns A& B on Sheets M and D, if A matches and B doesnt Copy B, else new row

Thanks for stopping by first of all. so I'll try to be Clear. I'm certain its simple for someone, but definitely kicking my butt this week.

I would like to compare 2 columns A and B in sheets Master(M) and Data(D), if the data matches in A, Check B, if B matches, go to Next. If Not, update Master(M) B with the values from Data(D) B, but if the values from Data(D) don't match in A or B, Add a new row to Master(M) from the unmatched row.

so Master(M) holds all the info, if Data(D) has a new listing in column A, add a Row in Master(M) with A and B from data, otherwise just Update B in Master(M) from Data(D)

I wound up altering other macros to temporally add the rows missing.. fortunately A1:A2 aren't confused with anything from the data sheet. Those are my header and statistics row. I do appreciate your help. that code helped me to move along at a pretty good pace on my crazy little project. Thanks again!