PC7 keeps trying to boot from usb HDD

Hi experts,

Ive just built my own machine, but it together with what seems like no problems, using a SSD drive for the OS and an external usb HDD for data. When i boot maybe 1/3 times it says i dont have a HDD with an OS, i got into the BIOS and its changed settings to boot from Removable Dev first and the SSD has gone. The only way i can get it to boot if this happends is to take out the USB.. boot to BIOS change the settings to boot to my SSD and then replug in my USB when im logged into Windows 7. These settings some times stay sometimes dont for the nxt time i shut down and start.

Its a ASUS M4A87TD Evo BIOS with Version 0501 on it.
http://www.ebuyer.com/product/206151


Also to get it to boot in the first place from BIOS i had to go to

Storage Configuration ->
OnChip SATA Channel (enabled/Disabled options)
SATA ports 1-4 (Auto, IDE, AHCI options)
SATA ports 5-8 (Auto, IDE, AHCI options)

OnChip SATA was enabled when i first went into the BIOS and the sata ports where on Auto. I had to change them to AHCI to get Windows 7 install Disk to see the SSD. But when it saw it and when i go to Choose boot order in the BIOS it shows up as IDE Corsair CSSDR disk. which puzzles me slightly?

Any ideas how to stop it freaking out when i have a USB HDD attached?

Thanks for any ideas help.

S