Can't boot without USB drive

I have read countless other threads on this problem and yet no solution worked for me.

I successfully installed with OpenCore 0.9.9 macOS Monterey on my desktop pc (Fujitsu Esprimo P556 E85+, chipset Intel H0110, i3 Skylake, HD530, 8GB RAM, HDD 500GB), everything works great. I then used MountEFI to copy my EFI folder (containing BOOT and OC) from the USB drive into the EFI partition on my HDD. But when I try to boot my pc without the USB drive, the BIOS (UEFI) doesn't recognize anything. I have no dual boot, just plain macOS on this drive. I looked in the BIOS and the HDD just doesn't show up as an option for booting. With the USB drive works like a charm.

I tried creating manually an EFI partition on the HDD (although, as I said, there's already the hidden one created during the process, which I mount with MountEFI), no luck.

Can someone please help?