Using the new NetBSD 7.1 amd64 cdrom,
I had a similar problem with an old HP Z400 system.
I tried to install NetBSD 7.1 but the installer failed on the newfs /dev/rwd0a ... and said that the hard drive was "read only".
The machine has no EFI and uses MBR for partitioning but
showed this bogus entry in the system log:
dk0 at wd0: EFI system partition
dk0: 102400 block at 2048 type msdos
To fix it I booted a slackware linux install dvd and ran this from the command line:
fixparts /dev/sda
Then type "q" for quit.
It removed an old gpt table at the end of my disk. Then I was able to install NetBSD with no disk problems.
fixparts is a program from the gptfdisk group of GPT partitioning programs.
http://www.rodsbooks.com/fixparts/index.html