mmm... Doesn't look good here ;-{
- Your [http://www.everymac.com/systems/apple/imac/specs/imac-core-i5-3.6-21-inch-aluminum-mid-2010-specs.html|systems] HD SATA port is SATA II (3.0 Gb/s)
- Your [http://www.wdc.com/wdproducts/library/SpecSheet/ENG/2879-701277.pdf|Western Digital WD10EZEX] HD is SATA III (6.0 Gb/s)
Your system can't handle the higher SATA speed HD. While some drives can be altered to the slower SATA I/O speed this one can't.
The older WD10EALS model could be: [http://wdc.custhelp.com/ci/fattach/get/251445/0/session/L2F2LzEvdGltZS8xNDE2MTQ1NDc0L3NpZC9WUXpoSHo3bQ==/filename/Caviar+Blue+Specifications+%28English%29.pdf|WD Caviar Blue]. Here's the info for it in case you can find it: [http://wdc.custhelp.com/app/answers/detail/a_id/1400/~/specifications-for-the-wd-caviar-blue-sata-6gb%2Fs%2C-3gb%2Fs-and-pata-100-mb%2Fs|Specifications for the WD Caviar Blue SATA 6Gb/s, 3Gb/s and PATA 100 Mb/s internal hard drives].
OK, you'll need to return the HD and try getting the older WD drive or, you could look at getting a drive which has the ability to 'Auto magically' set it's self to the correct SATA I/O speed. As an example the [http://www.seagate.com/www-content/product-content/barracuda-fam/desktop-sshd/en-us/docs/desktop-sshd-data-sheet-ds1788-2-1308us.pdf|Seagate 3.5 Desktop SSHD]. Here you can see the drive is able to run at SATA I, II & III I/O speeds. The only rub here is the special thermal sensor cable which you'll need to swap out for the Seagate part (if you go with the Seagate HD). Here's the different thermal sensor cables:
Cable, Temp Sensor, Hard Drive:
* 922-9215, Hitachi
* 922-9216, Seagate
* 922-9622, Western Digital
Or, you can cheat and use the Optical Drive 922-9624 thermal sensor cable depending on the drive you get. And just sticking it on the HD like the older models did.