HDD too small, can't mount without additional kits.

I recently bought a new computer, it came with a 1TB hdd which was mounted. I had a new 1TB HDD already, and when i tried mounting it, it was to small, the HDD that the pc came with had some plastics on it's sides that held it properly. I mounted the HDD and put it on the Case floor, it's working but recently i've been getting I/O errors, it all solved when i unplugged and plugged it again,*...