Which SSD drive should I use replacing my original HD

To start with I would leave the original HD in as long as it's working. If thats not the case I would look at replacing it with a new HD.

Now don't freak out here! Let me explain ...

Your system can support three SATA drives. Right now you have one port used for the HD, the second is the optical drive. Now we have one more! We can use this extra port for a SSD drive.

So here's the IFIXIT guide you'll need to follow to add in the second drive (SSD in this case): Installing iMac Intel 27" EMC 2429 Dual Hard Drive Kit. It is a bit of work but well worth it!

If you have a bad HD you could switch it out to a SSD which is a easier upgrade but you'll need some special parts. Here is the IFIXIT guide you'll need to follow: iMac Intel 27" EMC 2429 Hard Drive Replacement. Now you'll need these two parts besides the SSD (well talk about that at the end).

• OWC In-line Digital Thermal Sensor for iMac 2011 Hard Drive Upgrade
• OWC 2.5" to 3.5" Drive Adapter Bracket

If you decide to replace the HD with a newer HD as yours is dead you can go two ways here: a straight drive or a hybrid.

I would go with a straight HD if you are also planning on adding the SSD as a second drive as I've outlined above.

If you just want the benefits of a SSD but don't want to spend big bucks a good 1/2 step is a hybrid drive (SSHD). This is more important if you want deeper in system storage with a single drive and want the zip of a SSD.

Your current drive is a 3.5" SATA II (6.0 Gb/s). You can use any HD that meet this spec. For a SSHD I would recommend one of the Seagate Desktop SSHD's. For either a standard HD or SSHD you'll need the in-line thermal sensor listed above. as these drives are the same size as your current drive you won't need the bracket.

For a SSD using it as a second drive you can use any 2.5" SATA III (6.0 Gb/s). I personally like the Samsung 850 EVO drives Samsung has introduced a cheaper SSD line Samsung 750 EVO. I'm still evaluating them for our use.

Keep in mind the storage space of the SSD tends to be tight as the cost factor of the drives is steep. Which is why a dual setup is ideal as you get the benefits of each, the deep storage of your current HD and the speed of the new SSD second drive (what I had recommended at the top), without needing a big SSD drive (a 256 MB drive will do). Lastly, you won't need the parts I listed above you'll need this kit instead: iMac Intel 27" Mid 2011 Dual Hard Drive Kit.

If you go with replacing your current HD with a SSD then you'll need to think through how much storage space you can afford, the bigger the better here. 512 GB or larger. Just keep in mind 1, 2 or 4 TB SSD drives are available but they are very expensive!

Hopefully this explains your options and what you'll need for parts.