I have a source data spreadsheet consisting of 68 columns and about 10000+
rows. The unique identifier is in Column Q (not my design!). I'll call this
sheet (Source). Headers are in row 1.

I receive updates to the data in the form of a spreadsheet containing a
subset of Source with the columns in a defined format that matches Source,
and I need to perform a number of validation and comparison checks before
accepting these updates (lets call this sheet Update).

I would like to take the ID (say in Q2) from Update and check if it exists
in Source. If it does, I would then like to compare the entire row from
Update to see if it matches the corresponding row in Source. If there is a
mismatch, I would like the cell in Update to be shaded and a log (eg on a new
worksheet) detailing the ID, the Source value, the Update value and the
header row value.

I'm thinking that loading the row into an array would be the most efficient
and doing a loop to check each row in the Update sheet. However, I'm having
trouble getting started and would appreciate some pointers in the right
direction.

Thanks for your help. Please ask if you need anything clarified.