What kind of SSD I need to buy for my Mac

If your HD is too small for your needs now or having problems I would think hard here before swapping it out for a SSD. I would recommend getting a hybrid drive like Seagate's SSHD to replace the HD. Or, stick with a replacement HD and add in the SSD as I have outlined here which will give you the best performance bang.

Since it's an iMac, you can add the SSD on the third SATA port, clone your existing hard drive data to an external drive, reformat the HD+SSD as a Fusion Drive using Disk Utility, then clone the data back to the Fusion Drive. That gives you the combined space of the HD and the SSD, and is much faster than the stock hard drive. The SSD is a big cache for the Fusion system; data you use frequently (OS, apps, some user files) is automatically moved onto the SSD, while less frequently used files move to the hard drive. It's a hybrid hard drive, but with a much larger SSD cache (and probably a faster HD, if your current internal drive is 7200 RPM). Formatting a Fusion volume will involve erasing your internal hard drive, so be sure to clone your data first. You can also restore from a Time Machine backup; get a complete backup of your internal drive, install the SSD, create the Fusion volume, install OSX 10.9/Mavericks or 10.10/Yosemite, then transfer data back from your Time Machine backup during first startup.

Thank you for your answer Dan, it was very helpful. I'll try with the Samsung 850 EVO. Thank you.

If you want to use your optical drive space ,OWC gives you this option for $29.

iMac 2009-2011 Optical Bay Hard Drive / SSD Mounting Solution

Optical drive installation videos:

http://eshop.macsales.com/installvideos/...

http://eshop.macsales.com/installvideos/...

I highly recommend OWC SSD hard drives. They have up to 5 years warranty and pro level models from $97.99. With Blackmagic certification.

http://eshop.macsales.com/shop/SSD/OWC/M...

[image|619285]

You don't need this 12.7mm SATA Optical Bay SATA HD enclosure it is intended for the laptops or the older iMacs which don't have a third SATA port.