If you can't run EasyBCD without assigned disk letters, that implies it's looking for the BCD on one of those drives.
When you're up and running, which drive is flagged "system" in Disk Management.
Disk Management flags have the following meanings
"boot" = "this is the system you're...
The boot process
1.After pressing the power button, the PC’s firmware initiates a Power-On Self Test (POST) and loads firmware settings. This pre-boot process ends when a valid system disk is detected.
2.Firmware reads the master boot record (MBR), and then starts Bootmgr.exe. Bootmgr.exe...
I dual-boot W10/7 and the timer is set to 2 seconds and never varies.
EasyBCD is not active during boot, only MS bootmgr.
Anything you do with EasyBCD to change a BCD setting (like timeout) will take effect at the next boot and be permanent unless you or something else changes it again.
If you booted it and it was not C, then it was not a clone.
Check the documentation of the partition management software you used to create the copy.
There are normally many commands for making copies of a partition and some of them will modify the copy so that it is no longer a clone.
Or more simply, just remove the drive letter for the "other" system in Disk Management on the running OS, and vice versa when the other is booted.
Drives without letters are ignored (and hence inaccessible) in Explorer.
Sorry, stepping into this late-on, I saw the early EFI reference and have been assuming this to be a UEFI PC hence a lot of my previous comments which are irrelevant now that I can see that this is a legacy BIOS/MBR PC..
Looking back I see also that C is clearly marked as "system"
EasyBCD takes no part in the boot process. It adds, deletes ar alters entries in the BCD, and those changes will affect every subsequent boot, but it's MS bootmgr which is doing the booting every time.
What does "view settings" say is the boot device/path ?
From your earlier post #11, the presence of bootmgr etc seems to indicate that the partition in question is already "system". In UEFI, the active and system flags are no longer seen though "boot" still indicates which system is actually running.
You should see in Disk Management (though not...
You can only use EasyBCD on a running OS. It's a Windows .NET application .
If your PC can't boot, it won't help you.
Use your W10 Installation disc (or a W10 repair disc if you made one ( Control Panel > Backup & Restore > Create a system repair disc))
Boot it, select "Repair your Computer"...
Can you hear any beeps ?
If so, look on HP support website or Bing " HP beep codes" to find out what the number and pattern of beeps indicate.
That's the only way it has to communicate if it can't get as far as the screen driver.