With the installation of VMware ESXi 4, data stores will automatically be created using the default settings. This includes a block size of 1 MB. If you don't want that, you could delete the empty datastore and create a new one.

Sometimes, deleting the empty datastore fails with the message:

Quote
Call "HostDatastoreSystem.RemoveDatastore" for object "ha-datastoresystem" on ESXi "xx.xx.xx.xx" failed.

VMware's KB 1017104 provides the solution for another problem, but this works just as well with this issue, with a little variation.

Notice
Performing the steps in this solution will totally utterly destroy your data store. Do not use it if there's still any req  ...