let's say you are pulling data from HP array to VMAX.  Your host can begin to use VMAX devices even though data is still being pulled from HP array. If host does a write, it will write to VMAX volume ..and now all of a sudden HP array experiences hardware issue and your hotpull stops. At this point you probably want to restart hotpull but remember that write that you made earlier ..that write does not exist on HP array ..so the minute you restart hotpull, it will overwrite that write ..thus losing data. This is where the donor_update features comes in. Basically what happens is that as your host starts writing data to VMAX devices ..it will also write to HP array devices at the same time. So if hotpull stops again ..you can safely restart it without losing the data that was written to VMAX volumes.
I want to know more about the working effect of the option -donor_update, with symrcopy how does it actually protect from the SAN failures/connectivity during the OR hotpull migration ?,  Thanks,
-donor_update effect on Open replicator hotpull migration.