I have two external USB storage devices: * 16 GB USB stick (flash drive) * Old 2.5 inch 80 GB HDD I performed the following procedure with Disk Utility on both devices: * Delete existing partition. * Create a maximum-size new partition. Afterwards, Disk Utility's "Partition" tab shows for both devices that about 5 GB are not available: * 16 GB USB stick: 10.06 of 15.47 GB available (-> 5.41 GB missing) * Old 2.5 inch 80 GB HDD: 74.1 of 79.68 GB available (-> 5.58 GB missing) Since about 5 GB are unavailable on both devices and since 5 GB is about the size of the last few Mac OS installers (Yosemite, Mavericks, etc.), I wonder if having used these devices in the past to install Mac OS has created something like hidden partitions that cannot be easily removed. What do you think?