Chosen Solution
After a lot of research and a lot of confusion on the topic I decided to install an SSD on my mid-2011 21,5" iMac. At first I thought I would have to accept SATA II speeds from the optical bay. Then I realised that there are 2 SATA III ports, one of which is free (i’m adding an ssd but not removing my hdd). Then I searched on google for photos of these ports and I’m wondering… Can’t I just put the Optical bay cable on the SATA III port and suddenly make it a SATA III connector, instead of buying new SATA cables ? I wanna do that because I want to remove the optical drive and install the ssd there (not underneath the optical drive) at the same time I take advantage of the SATA III speeds. So, for short: If I unplug the optical bay from the SATA II and plug it to the SATA III connector, will the optical bay now have a 6gb link speed ? (So I can then replace it for an ssd) Thank you.
Yes, you can support three SATA drives if you want to (two SATA III & one SATA II). The optical drive cable would mate up with the special frame to hold the SSD with in it. Here’s the IFIXIT guide from the 2010 model which is the same process: Installing iMac Intel 21.5" EMC 2389 Dual Drive
A lot of unhelpful discussion here, but if someone is still wondering - yes, you can use the optical drive cable to connect to the secondary SATA III port of the iMac. The optical drive cable has two plugs - data and power. You have to reconnect the data plug to the SATA III port and leave the power plug in place.
