Whenever an attempt is made to change the partitioning on Mac disk by any Windows software other than HFS+ Disk Manager, Mac HFS+ Disk Support steps in to mediate the situation.This is done because if Windows software other than HFS+ Disk Manager makes changes to the partitioning on a Mac disk, the disk will no longer be Mac-partitioned or Mac-formatted, and all contents of the Mac disk will be lost.Sometimes this is a desirable outcome, if a Mac disk is no longer needed, and you want to repartition and reformat it for use exclusively with Windows. In this case, you should select Erase the Mac Disk.At other times, however, Mac HFS+ Disk Support steps in and protects a Mac disk from accidental damage. In that case, you should select Don’t Change the Mac Disk.If you decide that you really do want to change partitions on the Mac disk, but want it to remain a Mac disk when you are done, you should select Modify the Mac Disk.