If you need to upgrade a hp server with an iso file, such as Service Pack for Proliant or Intelligent Provisioning Recovery Media, you can not use generic software to create the usb key with the iso. You must use the HPE USB Key Utility for Windows / Linux.
Instructions for Windows
Download the file (link above) and run the installation.
In the c:\Program Files\usbkey folder, run the usbkey.exe file to create your iso on usb stick.