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I have an iMac Intel 20" EMC 2133 2.4 GHz Core 2 Duo Model A1224 which is having hard drive issues. I am looking at upgrading rather than replacing and trying to compare 3 available SSD drives which are all 500GB & pretty close in price: Crucial MX500, WD Blue, & Mercury Electra 3G. Any personal experience with any of these drives (or other recommendations) would be appreciated. My second question is: aside from the 3.5"–2.5" mounting adapter, will I need any SATA cable extensions to connect the drive?

Lets look at the question slightly differently here … What are the requirements the drive needs to work in this system. To start with the size is smaller than the current drive 2.5” Vs 3.5” so thats not an issue (you’ll need an adapter frame which you already know). The SATA connection is the same so thats not an issue and you shouldn’t need any cable extensions. So far nothing special … Now the fun one: The SATA I/O speed of your system is only SATA II (3.0 Gb/s) so what ever drive you get needs to support this I/O speed. Many of the newer drives are now fixed SATA III (6.0 Gb/s) speed and won’t work in your system so you need to be careful. As an example the Samsung 860 EVO supports all three data rates as you can see in the spec sheet here. Make sure the SSD you get states it, if not find another SSD. The OWC Mercury Electra 3G SSD is the only SSD that you’ve listed thats spec lists 3.0 Gb/s. Here’s the IFIXIT guide you’ll need to follow iMac Intel 20" EMC 2133 and 2210 Hard Drive Replacement Don’t forget to move the sensor over to your new drive at Step 19.