IRST may be used as both an AHCI and RAID driver, I have used it for both for years, that in itself is not the problem.
eBombzor, I have noticed that too with my 520's and IRST 11.6 and 11.7. Do you have a single 520 or a RAID 0 volume of 520's? I have two 520's in RAID 0, and noticed what you did. You can't access the 520 after you run the Optimizer for a few minutes, as in opening it to check folders or open files, correct?
You may be right about the new version(s) of IRST causing this. I thought it was related to the latest version of the Toolbox, and that I am able to run the Optimizer on a RAID 0 volume on my Z77 chipset board.
For example, you run the Optimizer, and it finishes quickly, correct? But then you can't exit the Toolbox or do other things. Then that stops in a few minutes, and all is well.
I notice that if I run the Optimizer on a different Intel SSD, like my 510 (non-OS drive), I can check files on that drive immediately, and no freezing occurs. So I wonder if this is related to the SandForce controller on the 520, since the 510 has a Marvell controller.
The optimizer in the Samsung Magician software takes about 4 - 5 minutes to complete, but I did not try to do other things while it ran. If the SandForce controller causes this, then Intel should have let it run for the few minutes the 520 is locked up, and then say the Optimizer finished, as Samsung does, IMO. I've read SandForce controllers handle TRIM differently than other controllers, as in they delay its application for use with idle GC. I wonder if Intel is forcing the TRIM to be applied, which causes the short lockup. Or, it is an IRST bug.