• Use Disk Utility to format the USB hard disk for use with a Mac (“OS X Extended (Journaled)”), if needed.

  

  Although USB disks are the most common external disks, you can use any other external disk that can connect to your server, like FireWire or Thunderbolt disks.

• Open the Server app to see the list of services available in OS X Server. Select Caching.

• In the Settings area, click Edit, then select the USB disk. Click Choose. Use the slider to set the cache size (it should be smaller than your disk capacity), then turn on the service.

  When Caching service reaches the cache size limit, previously cached downloads are removed to make room for new ones.

  

• Click the button to turn Caching service on.

  

After you set up Caching service, you can test it to make sure it’s saving downloads.

1. On a client device, open App Store, then check the update section. Install all available updates, and wait for the installation to complete.

   If you don’t have a client that needs an update, you can install an app of your choosing.

2. In the Stats pane of the Server app, check the Bytes Used amount to see the amount changed.

   If you don’t see cache space being used, make sure the client and the server are on the same local network.