HP Compaq Elite 8300 AIO restart issue.

[Quarry16]

Have a HP Compaq Elite 8300 AIO that has developed an issue when restarting. Every time the machine is restarted it acts like it has done a shutdown and does not restart. Instead, the power light blinks white once every 5 or so seconds and the fans inside kick up then stop, this behaviour repeats ad infinitum. 

To get the machine to boot I have to disconnect the power for about 10 seconds then reconnect and power on, the power light will briefly flash red before going white and the machine will then boot up as normal and not have any issues until it's restarted again or shutdown and I have to do the above procedure to get it to boot!

I've changed the HDD for an SSD but still the same issue, have also done a memory test (inbuilt which passed) and updated/reset the BIOS and also reformatted and installed Windows 10 Pro on the SSD (it did have Windows 8.1) from our VLSC portal but it has made no difference to the restart issue, it still acts like a shutdown and keeps trying to power on until the power is turned off for at least 10 seconds!

Anyone have any idea why this behaviour is happening?

[WAWood]

@Quarry16 

Win10 has enabled a new hibernation process known as Fast Startup -- which is enabled BY DEFAULT. This is different from Fast Boot, which is a BIOS/UEFI option.

Fast Startup is supposed to dramatically speed up the booting process, but in some PCs, it actually slows it down or causes it to hang. It's also a new form of hibernation known as hybrid sleep -- and this causes battery drain to maintain the state of the PC while turned off. In some rare cases, it can also cause a laptop to overheat because the processor is still running and still generating heat, as this has been reported when the laptop was turned off and put inside a case.

Disabling Fast Startup might fix booting and shutdown problems.

There are two ways to disable FastStartup in Win10: (1) through the Control Panel, and (2) through an elevated command prompt.

Control Panel - Open Control Panel --> Power Options.
Select "Choose what the power buttons do"
Select "Change settings that are currently unavailable"
At the bottom of the Window, under Shutdown settings, uncheck the box regarding fast startup

Elevated command prompt - run the following command:
REG ADD "HKLM\SYSTEM\CurrentControlSet\Control\Session Manager\Power" /V HiberbootEnabled /T REG_dWORD /D 0 /F

In both cases, reboot Windows.

NOW, Fast Startup is disabled.

See if that helps.

[Quarry16]

Fast Startup wasn't enabled, it's disabled by Group Policy, but that only affects shutdowns not restarts which is what the issue is with, restarts!