The reason, why the drive "show as an external HDD", I believe is because AHCI allows for hot-swapping drives, ie. you can replace the drive without turning the computer off (similar to an external drive in that regard). So it's normal.
That said, I don't know if there are any way to disable the hot-swapping feature, so that the drives won't show up there with the other replaceable media.
Amiga 1200, 68020@28MHz, 4MB+2MB RAM, Conner 80MB harddrive