Can I use this drive in this iMac?

There are a few issues here you'll need to consider.

• The given model you have may require a SATA/power cable harness for the HD/SSHD drive
• Opening the newer 'Thin Series' iMacs are a bear! And if you are not careful you can damage the display
• SSHD's can't be used in a Fusion Drive set so if you have an Apple Blade SSD drive you'll need to keep the two drives independent. Also make sure you break the Fusion drive before you take the HD out otherwise you have a very confused system.
• If you have a Apple Blade SSD you might want to think about just going with a bigger HD like a 1.5 TB or 2 TB SpinPoint HD

OK, you're going for it!

Make sure you have all of the correct tools and fully review this IFIXIT guide here: iMac Intel 21.5" EMC 2638 Hard Drive Replacement.

Now to the drive: Yes! The 2.5" 1 TB WD10J31X will fit and work in your system (review the spec sheet above).

But, are you sure you want to limit the SSD cache to only 8 GB? How about going with a different drive which has a deeper SSD cache (32 GB), it will give you better performance.

I would recommend going with the Seagate 2.5" 1 TB SSHD ST1000LX001