I might be mistaken here, but to my knowledge, due to the MBR being found on one of your sata HDD's (rather than your SSD) ...the boot manager cannot load the OS from your SSD drive when changing or moving the SSD to the top of the boot priority list as it does not contain the MBR.
If the MBR was found on the SSD, I don't think it would have any issue. This is why setting your HDD priority to one of your previous drives (sata drive) works, because the MBR can be found on this drive, therefore the Boot loader finds it, loads it and will then allow you to load the OS from either drive via the boot manager screen (which you wont even see if the machine cannot find or load the MBR - think 'the black screen' you mentioned after the asus startup screen).
Question - Did you install Windows 7 on your SSD WHILE your other drive(s) were connected?
I've had an issue in the past installing Win 7 on a sata drive however I left another drive plugged in and for some reason the MBR was defaulted to this drive (The Non-Windows HDD - probably to speed up boot performance). The problem only became noticable if I changed the boot order and allowed the HDD that I installed windows on, to boot up first - this was because the MBR was NOT found on this drive, but rather on the other one.
I simply pulled the data from the drive that had the MBR, re-formatted both disks and re-installed Win 7 on the one HDD whilst the other drive was disconnected. This then ensured the MBR was stored on the drive that contained Windows too...