This issue happens, typically, when you are using a RAID controller that is not a true hardware controller, but a RAID alternative known as ‘Fake RAID’. The Rocket RAID 1740 is a good example of a Fake RAID product, but there are many others which fall into this category. Reference the link below for more details.

What’s happening with a Fake RAID controller is that our system sees the sum of the sizes of the individual disks in the RAID, and not the size of one single drive, as it should. For example, if you have two disks of 750GB in your RAID, you’ll get a total capacity of 1500GB reported, instead of just the 750, thereby prompting the system to report the ‘over the limit’ error. If you find you have a Fake RAID controller in your system, the best solution is to replace it with a real hardware RAID controller, or add 2 x 500GB drives.  You can check with your manufacturer to verify your hardware, and/or check out the link listed below for a list of known fake and real RAID cards.