To start, I would like to say I have pretty much read every thread that coincides with what I am about to discuss and have had no luck with any of the answers/solutions suggested.
I have an Acer Extensa 5420 with Windows Vista installed. When I received the computer (second hand), I had an issue where the computer would start to boot and instantly say "Operating System Not Found". Instantly I thought, well lets try installing a fresh version of the OS. Installed and was working great until I removed the OS CD from the drive and restarted. Upon booting, it once again stated "Operating System Not Found".
I then decided, well maybe the hard drive is just dying, so I replaced the current SATA HDD with a new SATA SSD (figuring these drives are less likely to fail). Once again, seemed to work when installing the fresh OS on the new drive and things were working just fine. Removed the OS disk from the CD drive after the OS had been installed and upon booting the "Operating System Not Found" message once again appeared.
Quite confused, I turned to the BIOS where I noticed an "!" next to the HDD in the boot order menu. I had never seen this before, so I started to Google it. Arriving on this forum and various others, they all seemed to state that the "!" I was seeing either meant the drive was DISABLED or there was an error with the Motherboard recognizing the drive. Thinking that this laptop was built before SSD's were really well known, I swapped back to the original HDD. Going back to the boot menu in the BIOS, the "!" was still there (but now on the original hard drive I just swapped back in).
Upon further researching on Google, I found many different solutions being suggested. In the BIOS, select the drive and press "SHIFT" and the "1" at the same time, tried with no luck. I also saw, perform a hard reset, by removing the CMOS battery and letting sit (no luck) OR removing various other pieces of hardware and pressing a combination of buttons (no luck). I also moved the hard drive to the top of the boot list in hopes that being the first item, it would have to be enabled, but no such luck. After performing all those, still no luck. I can't find anything from Acer in terms of this problem and there is not BIOS firmware update that I can find.
I am currently out of options and from what I can tell, exhausted all possible solutions seen on the internet. The computer is no longer under warranty, so I can't do anything on that front and I am getting closer and closer to concluding that the issue lies with the motherboard and other pieces of hardware that are not easily/cost effectively replaceable.
I would also like to mention that I am a Computer/Electrical Engineer, so my knowledge of these things is pretty high, so don't hesitate to ask me complicated or technical questions to help you gather more info to answer my question better. Is there anything I am missing or have not tried? I an open to anything at this point.