I just couldn't let this wait until Monday ...
Phase 3 "Build Upgraded System"
- Confirmed stock drive 1 is still in slot 1 and system is off
- Set the new WD5000AAKB HDDs to "single" by removing their jumpers.
- Inserted new drive 1 into slot 2
- Started the system (Cold Boot)
- Confirmed everything was in working order and checked the Event Log.
- Confirmed the sync process has completed on the new drive in slot 2 via the Event Log.
- Shutdown the system
- Removed stock drive 1 from slot 1, label and allow it to join its comrades on the shelf
- Removed new drive 1 from slot 2 and reinsert it into slot 1
- Started the system (Cold Boot)
- Set the system to enter Recovery Mode on next boot. ("touch /nopivot")
- Reboot (Warm Boot)
- Preformed a fresh install of GOS 5.2.056
- Reboot (Warm Boot)
- Applied GOS 5.2.056 SP1 update
- Reboot (Warm Boot)
- Confirmed everything was in working order and checked the Event Log, then cleared the log.
- Shutdown the system
- Insert the "cleaned" new drives from Phase 2 into slot 2, 3, & 4.
- Started the system (Cold Boot)
- *Drum-roll* .....

- Phase FAILED
With two or more of the new drives in the system becomes sluggish plus the Event Log contains DMA timeouts and IDE bus resets. The initial GOS resync takes about 20 mins per drive, compared to the ~30 second resync with the stock drives. As long as I'm not transferring data to or from it, it works great ! haha
It seems others (via google) were successful using WD5000AAKB-00YSA0 (an earlier version) within other RAID enclosures, but were unable to get the later WD5000AAKB-00H8A0 versions to work.
I just happen to have the latter, so they will probably go back into their old FreeNAS enclosure where they had been working great as a tertiary backup for the last 1.5 years.
So what drives should I try? I've been able to find new Hitachi P7K500, Maxtor STM305004N1AAA, and Seagate ST3500630A OR I could roll the dice on some refurbished White Label drives. The Seagate ST3750640A are just to expensive for me to consider.
Most likely I'll use the stock drives until they fail, and put the system on a list for a rainy day mod project. I've been very pleased using Guardian OS for the last seven years and would like to continue using it at least for a little bit longer.
Once again, thank you for all your feedback.