"does not have enough free disk space" miscalculates disk space

My deployments failed today with the message:
A child activity failed: The drive containing the directory ‘C:\Octopus\Data\PackageCache’ does not have enough free disk space available for this operation to proceed. The disk only has 421 MB available; please free up at least 500 MB.

But the server drive C: still has 19 GB free disk space. Deleting files does not change the reported available space according to the error message. How can this be fixed?

The deployment worked after about 10 minutes of retrying. But it was still very puzzling why that happened. I had witnesses who saw what happened and we could not figure out why Octopus would report so much less disk space than was available.

Now I get it, I was looking at the wrong machine. Not the deployment target was the problem but the server with Octopus Server installed.