hdparm Write Cache automatically turned back on?

Hello,

I have a set of servers running PostgreSQL 8.4 and I'm building a second
set now. When I built them a few days ago, I turned off the Write Cache
using hdparm as described in the manual
http://www.postgresql.org/docs/8.4/static/wal-reliability.html Now, after
transport and rebooting Write Cache is back on. I checked my other servers
which had it off before and all of them have it turned on again too. I've
searched for the issue but haven't found anything. I wrongly assumed
setting the hard drive would be a one time setting.

Has anyone experienced this or know what might be happening?

The hard drives are WD 500GB black, the new servers are running the latest
CentOS 6.5 with latest updates. The versions of hdparm are below.

Thanks for your help!
Andrew

new:
Installed Packages
Name : hdparm
Arch : x86_64
Version : 9.43
Release : 4.el6

older:
Installed Packages
Name : hdparm
Arch : x86_64
Version : 9.16
Release : 3.4.el6