HARD DISC-changes

hello over there,
I have two HDD in my PC. All works fine. I bought a new DISK which is bigger than my first one and I thought I was clever enough, FDISK- all went well, formated and installed my OS on it. Now, I just changed this disk with te one which was originally there(all connections ok). It boots well but cant read my drives on the second DISK(although it recognises it).
How can I make this work out?.
NB:- It assigned my CD and Burner ,drive letters which correspond to my drive letters on the second DISK- could this be the problem? or…
Any useful help will be appreciated- thank you