fusion drive is dead in my 27inch imac 2012

Hi, i have a 27inch imac (late 2012). 3 days ago, my imac started to have problem. Everytime i login, the computer very slow and very unresponsive, most of the time i saw the spinning wheel. Today i got couple of error message , said "disk utility can not repair Data partition" ... or something like that, i don't rememeber the exact words. My mac , as far i remember , has 1 tb fusion drive. I create 2 partitions : Macintosh HD (system) and Data. So i was thinking something wrong with the data partition. I can hear the noise like hdd trying to spin over and over again. I couldn't do anything with this data partition , as everytime i login , i just freezze. I tried to go to safe mode , but still the same.

So i then restart and go to recovery partition (using cmd+R when booting). I have the timemachine backup and installation disk already prepared. So when in the recovery boot, i tried to inspect the drive and try to repair disk and got message said that the disk utility cannot repair bla bla.... Then when i tried to click the macintosh HD or Data partition (in the disk utility list) , suddenly a window popup ... i didn't remember what the message , but it said i need to rebuild the disk. Since i have the backup in my timemachine, i don't mind to rebuild. The rebuild process freezze couple of time . Maybe the 3rd time i reboot and go to rebuild again, finally i got my Macintosh HDD back in the disk utility. I check the drive and got no error... but the info said , i got only 120GB rather than 1TB. From this information i suspect that my mac is using fusion drive. I don't know what the size composition of the SSD + HDD in my fusion drive, but i think this 120gb is my SSD and i lost my HDD. Am i right ? what should i do now ? let say i want to replace the hdd, what hdd is compatible with my imac 27inch i7 late 2012 ? after put the HDD inside , what should i do to setup the fusion drive ?

Anyone can help ?

Thanks