Precision 5820, flashing light on the front panel

That is activity indicator light.  You can monitor the Performance tab (in Task Manager) and observe for any spike in each of the component graph that may coincide with when the light blinks.

Narrow down your troubleshooting test with storage and network:

You can test storage by removing all drives except boot drive and observing for changes.  

You can test with add-on NIC removed and observe for changes.

@Chino de Oro​

Thanks for the input. This (random BSODs, flashing light, speed drop in both internal and external transfer rates, delays, and more) is a relatively new issue, the machine has never behaved like this before. This kind of flashing light can't hardly be just a normal activity indication?

The storage devices has been present onboard since I configured this system a couple of years ago, and there are no programs set to run out of them. Malwarebytes and ZoneAlarm have their own live threat emulation running in the bakground, so I very much doubt that a malware can sneak in commands undetected. Also, it would hardly flash the light to advertise the user if it did...

Shifting/disableing the two network controllers, one at the time, is on the top of my ”done”-list above. It didn't help.

Agree, the constant blinks of activity indicator light is certainly not normal.  You also have done extensive steps for troubleshooting as well.  Therefore, I only added a couple suggestions.  While you monitor the Task Manager, you might match up the light blinks with activity spike from storage drives and network.  Then verifying with the services to learn which task just ran.

The reason I don't think blinking light as message PC was trying to convey because the pattern of LED light codes are preprogrammed pre-boot.  You may find better message from PC by review the event viewer.

Although, your post is mentioned about the blinking light for now, if you work on resolving the BSOD, it may affect and resolve the blinking light issue as well.

@Chino de Oro​

Indeed, the Task Manager is getting updated at about the same rate as the flashing light, but the two are not quite syncronised. Besides, keeping track of which service (out of 50-60 that are constantly jumping around) is altered at which spike requires an AI's observation capacity! I've tried but was forced to give up because of the risk for cerebral hemorrhage... :-\

You are of course right about the pre-boot error blinking patterns, I didn't think of that. Unless of course this kind of blinking is a secret message from the Devil, telling me that - ”your computer is just about to be <Profanity is never allowed. DELL-Admin>!” :-\ Seriously, I have tremendous problems accepting these blinkings as some kind of program behaviour, all the circumstances seem to contradict that.

The Event Viewer – been there, done that. I've been in touch with both Dell and Intel support, and I have an open ticket with CheckPoint (ZoneAlarm) support, trying to figure out what exactly is the Event Viewer and the BSOD-dump logs trying to tell me. The standard suggestions I've got so far (run the pre-boot diagnostic tests, or to reinstall the BIOS) gave nothing, while the most drastic one, to reinstall Windows, is on hold until I get desperate enough.

I'll will probably have to start a new thread under a different subject line for the rest of my problems. Thak you for your suggestions!

Load the hw test in the bios area and check the disks. It looks to me that you have a failing drive.

Alternatively use crystaldiskinfo ( freeware ) and check the smart values and status

Crystal Dew World [en] - (crystalmark.info)