It looks to me like the issue is that your column offset is 37.
- The column offset indicates the number of columns to skip before writing the numeric data. col is zero-based, so that col = 0 instructs MATLAB to begin writing in the first column of the destination file. Skipped columns are separated by commas.
By telling it to skip 37 columns, you get 37 commas with nothing between, and it starts writing the 38th column. However, you only have 37 columns of data, so the result is a csv file with nothing but commas. See here.
Try this instead
Also note that both of these examples will give you a single file. You would need to have code to create the file of the first 850k rows.