Not the easiest thing to do, but way faster now.
Il Mio Problema
Broken glass impelled me to buy a replacement and iFixit tool kit 2 years ago. Then I read about the iFixit SSD harness, so... put in a SanDisk 128 SSD with IFixit hard drive. Problems, problems.
My issues with the earlier installation were:
1. bent / broke display data plug teeth twice reinstalling —ambient light, case slipping, fat hand all contributory factors. (Repair guy had to finish it for me — $150.00 plus 2 extra cables.)
2. 128 SSD kept running out of space, was corrupted regularly. Constantly had to use Disk Utility and then MacPaw CleanMyMac 3 to jolly it along. Two different configurations, first I paid for (guy told me Fusion Drive wasn't really necessary), second I got off line about 6 months ago. Problems decreased but never went away.
Reminded me of my 1966 USAF experience with a giant room-sized Univac — it was down about half the time.
La Mia Riparazione
Replaced 2428 internal WD 500GB with a Seagate 1TB hybrid hard drive. I girded my loins for this latest foray into the guts of my 'puter.
Lots of research, but I can never lock in a complete step-by-step list of parts, changes to SUDO, etc. So I'm glad to report that my girlfriend showed up at the end of the day with steady, small hands to hold lights and reconnect the LCD thermal sensor cable (my day was spent straightening the tiny pin on the right without breaking it — although the metal cover on LCD plug bent backwards, then broke).
Then she helped with that pesky Display Data Plug — angle of insertion, that pull tab and a sinking lack of perception about whether it's seated properly being my previous downfall.
Il Mio Consiglio
1. Note to self: Losing 2 T10 8mm Torx screws for reassembly not real brilliant.
2. Fan speed flat out ("SMART" button didn't show up for me using that first recommended utility, but turning iMac off, unplug, waiting etc, then reboot worked - fan still seems noisy but it's not constant.
3. Make a Fusion Drive of your SSD + HDD with Terminal —
A. diskutil list —> to find the mount points for both drives;
B. diskutil coreStorage create FUSION /dev/disk# /dev/disk# —> where # stands for those two different mount points;
C. diskutil coreStorage createVolume lvgUUID jhfs+ “Fusion HD” 100% —> where lvgUUID = that big long serial number that showed up after the last command; AND FINALLY
D. sudo trimforce enable ... WOW! UberSanDisk was fast before, but FUSION HD 100% SCREAMS.