SSD working via USB but not if it is installed into the MacBook.

When I turn on my MacBook with the SSD plugged in, I can boot macOS from it by going to EFI mode and then choosing that SSD. If the SSD is installed internally into the Mac, I cannot enter it through EFI, because it just leaves me on a stuck apple logo OR it flashes quickly an apple logo and then takes me back to the EFI menu. Anyone knows what to do?

Modell: MacBook Pro 8,1

submitted by /u/S4NGM4USI
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