HD boot priority keeps swapping itself...

Ok, hey all...new here and wondered if the good folks here could possibly offer me any advice on the issue I am having. I currently have Windows Home premium 64bit installed on my SSD, but I also run a 1TB Westernd digital HD to store things on. Every now and then, probably once a week I get the old bootmgr is missing error. I know how to fix it using the install cd, but what I have discovered is that the error is caused, atleast I think so, by the hard disk boot priority switching around. Instead of the SSD with the OS being the first boot, it has switched and now the WDC HD is the first boot. Because there is no OS installed on the WDC HD it throws up the bootmgr is missing error. All I then do is swicth them back so the SSD is the first boot, save restart and it boots fine. My question is why would it be switching them both around every now and then? I don't swap any hardware or update software out before it happens, it just seems to be a totally random thing. Any advice, help would be greatly appreciated as it is starting to p!ss the wife off as it is primarily a work system used for her photography.

Thanks in advance,
Mark

Oh, thought I might include some info from the setup screens for the boot priority etc.
SCSI-0 is the SSD with the OS on it
SATA 3 is the WDC HD without the OS on it.

The above two is where the switching happens.
First boot is the hard disk
Second boot is the Pioneer DVD
Third boot is removable
Boot from other device is enabled.

Have you tried clearing the CMOS by going into your BIOS and resetting it to default settings? If not, How To Clear CMOS (Reset BIOS) may interest you. Clear the CMOS, then set the boot order to the order you prefer. Note: If you are using a RAID controller, this will reset the RAID and you will lose your data. Backup all important files and make a system image before proceeding with this step if you have multiple hard drives in a RAID configuration. If you are unsure whether you are running a RAID configuration, do the following steps.

1. Click Start Menu
2. Right Click My Computer/Computer
3. Click Manage
4. Click Device Manager from the list on the left
5. Expand Disk Drives and check for RAID manufacturers (use Google or ask us by typing in manufacturers listed in your Disk Drives list)

Alternatively:

1. Login as an adminstrative user.
2. Click Start Menu
3. Click Control Panel
4. Click Hardware and Sound
5. Click Device Manager (the last link under Devices and Printers)
6. Expand Disk Drives and check for RAID manufacturers (use Google or ask us by typing in manufacturers listed in your Disk Drives list)