Question : Problem: ASUS A8N-SLI SE "Non-system disk or disk error. Replace and press any key when ready."

I installed this system board to a customer back on November 2006 last year. The customer have error message: "Non-system disk or disk error. Replace and press any key when ready."
Each time I changed the Bios setting and it is all right.
Recently, my customer was away from his computer for about 2 weeks and when he comes back the computer was working all right for a few days. After that he has lost his BIOS setting again. I have update the BIOS and the problme still happen from time to time every few months
I would like to see if there is any suggestion.
I have contacted the ASUS UK and they asked me for 15 pounds 12.00 for postage and insurnace after repair. Including the postage to ASUS then it would more then the value of the system board.

Answer : Problem: ASUS A8N-SLI SE "Non-system disk or disk error. Replace and press any key when ready."

To CiTech05
I do not understand what you mean with "XP and Asus boards - Has been known to move the ntldr to an external drive if plugged in during startup. I have seen this on ASROCK and my current ASUS board. Fixboot in recovery console usually fixes this." (it run in XP)
HDD is OK as I have chek it.

NTLDR is looked for to boot from the HD, if not available, it will give no os found msg after post.
On my ASUS and their sister company, ASRock (I own an A8N and an eSATA2), if a USB drive is plugged in during post, no matter the boot order, will move the NTLDR to the USB and give you the error described.

If it only happens on rare occasions, probably not the issue, as this happens on mine every time consistently. No workaround for it on either boards (ASUS doesnt see it as a problem so the BIOS hasnt been fixed).
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If it's a power issue, likely since it takes so long for the issue to come back, I would lean more towards a failing PSU (Power Supply) than a faulty connection. I have only experienced poor connections that would create inconsistant and various failures during usage, in other words, not total failures at regular intervals as you are describing, although it is possible. It is also possible that just that line from the PSU to that connector is going.
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Failures at extended intervals could be a heat issue with the PSU or HD.
Dirty power could also lead to increased heat in the PSU and failure months out.

When it comes back in, you can try looking at the SMART data to see if any failures are recorded. Seagates utility may not find any current issues but if there have been any problems over its lifespan (unless disabled) it should be recorded in the SMART tables.

*If it is currently working (since you have redone the BIOS settings and sent it back to the customer) than we can throw around ideas all day, but wont know if anything works for a few months until it would be expected to go down again.