OC: Cannot boot from internal drive when connected via SATA

Hi,

I was following Dortania's guide for Big Sur installation on my Fujitsu Esprimo P720 and it worked perfectly. But now in the post installation phase I am having trouble booting from internal SSD. I moved the EFI folder from the usb to the EFI partition of the internal SSD, but there are no boot entries when starting. When I connect the SSD via USB however, I can see the boot entries and Big Sur boots as expected.

Do you have an idea on how to solve this issue?

Hardware:

CPU: Intel i5 4590
Chipset: Intel 8 Series/C220
GPU: Intel HD4600