Yes Mint8 is Grub2
However when connected to hers Grub gets confused
What exactly happens? Does the drive just fail to boot, or does the grub menu come up but fails to load Mint?
Installed on a IDE drive connected to a eSATA port.
To begin with her BIOS doesn't support USB-HD so I had to connect directly to the IDE ribbon cable.
The problem could be that when you installed (on your computer) the drive was the second drive on the computer but when you put it in your sisters computer it is being recognized as the first drive. So the drive references in grub.cfg are trying to find a drive that doesnt exist. Without more details-- depending on your setup when you installed, it is possible that when grub was installed to the Master Boot Record, it installed to the MBR on your copmuter's main HD instead of the USB drive. In this case the drive youre swapping to the new computer doesnt have a MBR and wouldnt be able to boot.
Along the same vein, if your sisters computer still has its 1st HD and your putting this new one in as the second, then the MBR on the xisting drive doesnt know anything about the new one- in that case it would boot up the same as it used to without the new second drive.
Depending on what the problem is it can be fixed by re-installing grub to the MBR or adjusting the grub.cfg file. HOWEVER- youd likely need a live CD to make the changes, and given that the whole install only takes about 30 minutes, the easiest solution is probably to install the new drive and then just install Mint 8. Unless you did a whole lot of customizing after the install or there is some other reason that justifies taking the time to fix grub.