Recently I've had a few problems when I power on my 3620 but usually it has powered on after a couple of attempts and so I haven't bother too much about it. Normally I leave the machine running 24 x 7 as it is doing research processing for the World Community Grid, so its not often that it gets powered off and then on again. However this morning it was running particularly slowly despite having an SSD and 16 GB of memory so I decided to power it off and start again from fresh. BUT it wouldn't power on.

After a while I wondered if the very small flashing yellow light on the power on button was trying to tell me something - it was. Apparently this led can flash yellow or white and the timing and number of flashes are important.

So I watched carefully and it seemed to flash twice, then pause and then flash once again. I looked this up in the 3620 maual and it said there was a system board problem!!! Oh dear. 

After thinking about this for a while I wondered if it could be power supply related since I really did hope the system board had not gone kaput (by the way there are supposed to be different combinations of flashing lights for power problems). So I tried pulling out the mains plug a few times (I couldn't get it to power off just by pressing in the power on/off button) and pushing it back in. I tried waiting a while, including pressing the power on/off button with the mains cable disconnected to drain any power from the capacitors but nothing changed.

Then I though I would try disconnecting a few devices - I have a lot of USB attached devices including a powered USB extension device with 7 additional USB sockets on it. As luck would have it this was the first device I tried to disconnect - lo and behold there was an instant response from the machine and fans began to turn and it just powered up. Once the system was up and Windows 10 was running I reconnected the powered USB extender device and everything continued as normal. It would seem that for some reason it was the attachment of this device that was causing the system not to power up and giving a false message about a system board problem! So if you have this power on problem with a double flash followed by a single one you might try disconnecting your USB devices in turn (while the power on/off lights are still flashing) and see if your machine will restart. Your mileage may vary. Good Luck.