Grub installs in two parts. One part is in a Linux partition in a file called /boot/grub/menu.lst. The other part is a 512 byte record on the MBR which points to the correct menu.lst. When you formated the Linux partition menu.lst disappeared.
When you removed a drive the BIOS began booting the MBR on the other drive which apparently is a Windows MBR.
Your challenge, my lovely panel of generous and helpful experts, is to tell me how I can remove the busted GRUB loader without losing any data on that drive.
You already lost the data on that drive when you formated it with diskpart. The only part of that drive which is still intact is the Grub MBR.
The standard practice in data processing since about 1945 is to back up everything periodically and recover lost data from backup. Microsoft has introduced the weird idea of not backing up and instead trying to recover lost data by rummaging around through the disk control blocks in the hope of finding the lost data still intact in the disk free space. So you can either restore from backup or try to recover it with the Windows commands which rummage around disk free space.