However, theoretically, attacker can boot the laptop and disconnect the disk to get a decrypted disk.
This is not how this works. The bits are never decrypted on your disk / partition. If that was the case, a power loss would leave your device decrypted as well, and that is something a potential attacker might have control over.
What actually happens is that encrypted data is read from disk by the CPU, decrypted by the CPU and then written to RAM unencrypted. Unencrypted data should NEVER be written to non-volatile memory (the necessary exceptions, like the boot image, apply).