Upgraded my iMac 2010 11,3 27" from old 1TB to 2TB Seagate SSHD.
As it's a 2010 model, a special connector had to be purchase. Got the entire change out kit from OWC.
1) OWC HDD Upgrade Kit with tools & in-line thermal for all Apple Late 2009-2010 iMac 21.5" and 27" Models. Upgrade an iMac with any SATA Drive. No Software hacks, AHT Diagnostic compatible. Hard drive sold separately. 1 Year OWC Limited Warranty.
$53.99
2) Toshiba Canvio USB 3.0 Enclosure for all 3.5" SATA Hard Drives. Power supply & USB 3.0 cable included, just add your drive. *Open Box. Original Drive Removed - See Extended Description regarding enclosure clips.* 90 Day OWC Fulfilled Limited Warranty
$7.75
3) 2.0TB Seagate Desktop SSHD 3.5-inch SATA 6.0Gb/s (3.0Gb/s and 1.5Gb/s backwards compatible) 7200RPM Hybrid Drive with 8GB Flash; Seagate 3 Year Limited Warranty.
$108.00
Total: SGD285 (inclusive of shipping) , DIY. worked like a charm!