Be sure removing all partitions and format a FAT32 partition on the USB stick by using the following command:
Open the command prompt and enter the following commands:
diskpart (make sure you run diskpart as administrator) list disk (list the disk in your system including the USB) select disk USB number clean create partition primary active format fs=fat32 quick assign exit
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Click Yes when a popup box appears with the question to overwrite an existing file.
In less than a minute you will have a bootable usb media with the vSphere 5 installation files on it!