You need to go into the BIOS and set the HDD device priority as DSTM mentioned.
Depending on your MOBO (Motherboard) the SATA ports are labelled 1,2,3 etc. If you are putting your 'old' drive in to say 1, when your other is in 3 it will boot 1 first, if that makes sense. Also when you have more than one HDD installed (regardless whether SATA, IDE or SCSI)
there is a list, you need to check that the 'new' drive is at the top of that list.
Hope this helps, when you make amendments in the BIOS always remember to SAVE and EXIT