I have 2 spreadsheets - a master spreadsheet and sheet #2. Sheet #2 contains
all of the information from the master (with the same exact columns), plus
new rows that are inserted throughout. The first column of data uniquely
identifies each row. Most of the rows are identical in both, with some
tweaks to the master. Therefore, I do not want any cells in the pre-existing
rows to be overwritten in the master.

I'd like to merge in just the new rows from sheet #2 into the master, and
leave everything that is already in the master (and has been tweaked), as is.

How do I do that?

Thanks.