Why does upgrade from Red Hat Enterprise Linux 6 to 7 fail with error "IOError [Errno 28] No space left on device" ?

Issue

• Upgrade from Red Hat Enterprise Linux 6.9 to 7.3 fail with the traceback as follows.

Traceback (most recent call last):
  File "/usr/bin/redhat-upgrade-tool", line 432, in <module>
    main(args)
  File "/usr/bin/redhat-upgrade-tool", line 299, in main
    kernel, initrd = f.download_boot_images() # TODO: force arch?
  File "/usr/lib/python2.6/site-packages/redhat_upgrade_tool/download.py", line 448, in download_boot_images
    copy2(initrd, initrdpath)
  File "/usr/lib64/python2.6/shutil.py", line 95, in copy2
    copyfile(src, dst)
  File "/usr/lib64/python2.6/shutil.py", line 52, in copyfile
    copyfileobj(fsrc, fdst)
  File "/usr/lib64/python2.6/shutil.py", line 31, in copyfileobj
    fdst.write(buf)
IOError: [Errno 28] No space left on device