Starcraft 2 – Why Choose 9-Overlord Over 10-Overlord

Against Terran specifically, you should attack the SCV as it is building since that is the weak link. It will go down quickly and then you will not have to worry about it any more.

Zerg players using this tactic will often cancel the extractor as it gets close to finishing and start it over again, to maximize how annoying it is since the health of the extractor resets to maximum every time. In this case, at least he cannot grab both geysers.

Protoss players can build assimilators on both geysers fairly easily, but it's at a cost of 75 minerals each so you will be able to get out an extra zealot and a half that he won't have access to, giving you a slight army advantage first until you get the assimilators away.

Overall, the main thing you have to do against this kind of tactic is make sure that it doesn't affect your macro. Keep building probes nonstop to mine minerals as fast as possible, and possibly go for a 2-gateway or more build to pump out zealots as fast as possible until you've dealt with the issue.

I'm going to drop some statistics and draw conclusions from them:

• Barracks -> 150 Minerals
• Minerals Harvested by a MULE over its life time -> 240-270
• Cost of a Supply Depot -> 100 Minerals

Ignoring the fact that a Scan also provide detection vs invisible units, the Orbital Command basically has 3 possible uses. I can use it to Scout; I can use it to build supplies; I can use it to Harvest. You'll notice that it is that harvesting that tends to provide the most supplies. In fact, even when we take the build time of a Barracks into account (60 seconds) and assume the SCV would have been able to get minerals during its build time (which is not true in a saturation situation), it is still more cost effective to build a Barracks and fly it over than to waste a MULE on a scan.

Now I'm not advocating never scanning; the ability to detect invisible units justifies keeping extra energy on hand for this purpose; but you should consider the implications of not using a MULE. Because MULEs can harvest from patches while an SCV is harvesting from them, your scan is essentially costing 240 Minerals.