Harddisk exchange from one MacBook Pro to another MacBook Pro with same configuration and model with different serial no
I'd suggest putting both the harddisks back into the devices they were originally in and try booting again. Make sure the laptops are plugged in when you do this. If that doesn't work, it's very possible you've damaged the disks by trying to pull them out while the laptops were running.

If it does work, then I'd use time machine to create a backup of the 100GB disk you're replacing on an external USB drive. Then, after that's complete, power off the laptop, replace the disk with a new one, then follow the steps here to "Restore from a Time Machine Backup".

To be clear, this is assuming these are older MacBook Pro's with removable harddisks, not the solid-state drives that are soldered onto the motherboards. Those cannot be removed or replaced, at least not without permanently destroying the entire machine.