Your error is not related to grub, but to the Debian initrd system. After grub load the initrd file system and the kernel, it run the kernel on a ramdisk with initrd. This initrd system include a series of command that should load you system.
One of the firsts step is to load all drivers, check for all hardware usable for booting, then looking for the device passed as ROOT parameter when grub started the kernel.
In your case, this ROOT device is not present, so you have a few options: one of them may be to extend the period the kernel wait in order to check for all sow devices. I don't think this is your case. So i assume you have some problem with your LVM configuration, and suggest to remove the new hard disk.
It is not clear if you expanded just the LVM group (adding a new phisical volume to the group) or if you also extended the LVM volume of root. In the first case, you may boot from a CD, reduce the group removing you new hard disk, and reboot.
If you extented your LVM root volume and resized that file system, you have to shrink the (unmounted) file system, then reduce the volume, then reduce the group and remove the new hard disk.