Windows 7 - Dual-boot :ubuntu on PATA disk, 7 on SATA disk

I have 7 installed on 640 GB SATA drive
Wanna install ubuntu on 80GB PATA drive
Yet the problem is that I can't make the SATA drive the first boot drive in BIOS.
My MOBO is GA-MA74GM-S2H

Are you setting in Disk priority or Boot Priority? Whichever drive is set first in disk priority will show up in Boot priority. Of course, different bioses handle certain things differently.

On Asus boards once you are in the boot section you simply click on "hard drives" to bring up another screen showing the list of hard drives installed. The one at the top is the one set as the default HD. The + and - keys on the Numpad are used there to move one or another drive to the top once highlighted.

The download page for the board manual if you don't have that onhand is found at GIGABYTE - Support&Download - Motherboard - Manual Download

Chapter #2 goes into the Integrate Peripherals on page #42 there. Page #40 shows the hard drive priority item first there however. Once you set which is the default HD you then set hard drive as the first.

Suppose I do the install as b4, BIOS will boot PATA HDD at first with GRUB boot loader.this way,7 will boot easily,right.

I had a previous problem with this sequence. I installed on SATA, Boot folder was on PATA drive[PATA had no OS] ,but when I detached PATA, 7 won't boot.
Will this happen again, if so, will what u have mentioned solve it?

When any PATA or otherwise called EIDE or ide for short is present other then an optical drive the Windows installer will go there first to plave boot files and entries into the master boot record(mbr). By unplugging the ide drive you may be able to repair the boot information with the startup repair tool found on the dvd. That would make the sata drive bootable and a stand alone drive there.

Once the sata is bootable you would unplug that and replug the ide drive back in to install Linux there without seeing Grub installed into the Windows mbr. The next step once the 7 sata is replugged is to boot into that and run the EasyBCD tool that features NeoGrub and make to check off the "Grub isn't installed into boot sector" option to add the distro into the 7 BCD store as your new OS choice at startup.

A few things here to note are if the startup repair works 7 will be seen as (recovered) in the boot options menu. With more then one sata without an ide drive present the Windows installer looks for the first sata HD to place boot files and mbr entries there.

Another thing to note once the 7 drive is bootable and the ide drive is replugged at least with the 8.10 and previous releases of ubuntu you could install the distro onto another drive or partition without seeing Grub installed in the Windows mbr. The manual not automatic partitioning option which wants the entire drive would allow you to edit the root partition information to see that made the mount point "/" where you then point NeoGrub at that.

If you have the PATA drive with the active partition and the boot files, primary, it will boot to that.

If you make the SATA drive primary or remove the PATA drive (same thing basically) without the boot files, it will not boot.

If you are going to disconnect or reverse the primary drive, make sure the partition of the SATA drive (Win 7) is active. I you do not, Startup repair might run for hours as opposed to seconds.

Once you simply unplug the ide drive before proceeding with the installation the Windows installer will make the sata active as part of the process along with placing the boot files there making the drive bootable to start with. Later once the ide drive(XP) is plugged back in you simply copy the XP boot files over to the root of the 7 drive(boot.ini, NTDETECT.COM, ntldr) and add the new XP entry into the 7 bootloader when having that drive set as the default boot device.

To see that done you first have to unhide all protected system files by unchecking the boxes for "Hide protected operating system files" and "Hide extensions for known file types" along with a few other items in the menu bar>Tools>Folder options>View" tab and a need to edit the boot.ini file itself to have the "rdisk(0)partition(1)" value changed to "rdisk(1)partition(1)" seen in the default OS line and underneath in the operating systems list which will only show XP there.

You will likely need to right click and uncheck and apply the read only box to see that done first on the copy of the boot.ini file. From there simply use a good tool like EasyBCD to add the XP entry in. No Fuss! No worries! Once added you can set which OS will be default with EasyBCD there as well as in the "Control Panel>System>advanced system settings>Startup and Recovery" section.