I have two HDDs, one very old 160gb that holds non-essential data that i've just been migrating from each new system onwards and a newer 500gb.
The 500gb has my main OS, windows 7 installed on it as well as Ubuntu on another partition. The 160gb has an old vista install that is never used.
The last week or so, SMART has been giving me errors that my 160gb is close to failing so I am in the process of getting the data backed up, however once I try and remove the 160gb from my machine I am unable to boot into windows7 or Ubuntu.
When I have just the 500gb in, it wont boot (it looks to me like it can't find a bootloader). When I have just the 160gb in it, it gives me a GRUB (GRand Unified Bootloader) failure command prompt type of thing, obviously because it cant see the OS located on the other drive.
My question is how can I eliminate this 160gb drive from my system (how can I get the 500gb to have its own bootloader, are there any programs out there I can use?).
Some other info that might be useful is the win7 was installed first, followed by the Ubuntu and when I boot the computer, it defaults to Ubuntu unless I choose win7. After I choose win7 I am givin the option to either boot win7 or vista (I had at one time dual booted those OS's shortly after installing win7.
I could answer this with certainty if Ubuntu wasn't involved. I'll post that answer, and you can see how it applies in the mixed-OS world.
With Windows, if you have a bootable older-OS drive in the system, put in a new drive, and install a newer OS, the boot part of the newer OS is installed over the boot part of the older drive. At this point, you need both drives to boot, which sounds like your issue.
(This is why the guides to building Win7 on an SSD all instruct you to remove all other drives for the install).
For Win7, the solution is to unplug the old disk, boot off the Win7 DVD, and do a repair install. This will see the Win7 installation on the only drive, note that it lacks a boot loader, and install said boot loader. If you were to do this you would be able to run Win7 from the new drive, but I have no knowledge of how to do the same thing for your Ubuntu installation. If you figure it out let me know, and I'll learn something.
Sounds to me like the MBR might live on the smaller older drive. Doing as WyomingKnott suggest should create a new MBR on the larger drive. Another approach would be to move the MBR. Suggestions in the thread below.
Moving might be a better option, since having 2 MBRs is a recipie for mishap as you can probably guess. Diskmgmt.msc into a run box. That'll get you half way. I would imagine this can also be done with Ubuntu's disk management suite, though it isn't one I've ever used.