The generic VMware ISO works on most whitebox hardware. But for vintage Dell or HP servers, always use the vendor-specific ISO . The generic ISO lacks proprietary drivers for RAID controllers, resulting in “No disks detected” during installation.
Once you've downloaded the ESXi 5.1 ISO image, you'll need to create a bootable USB drive to install the hypervisor on your host machine. You can use tools like (on Windows) or Etcher (on Linux) to create a bootable USB drive. download esxi 5.1 iso image