It appears someone replaced your HDD at some point with an old SATA II (3.0 Gb/s) laptop drive! The original HDD this system had was a SATA III (6.0 Gb/s)
So as long as you put in a SATA III (6.0 Gb/s) SSD it will interface with the system at the higher data rate. Make sure you review the given drives spec sheet.
'''FYI''' - This explains your current problem as the Fusion drive set was not designed to work with the older SATA II HDD’s as its not fast enough!