So I recently installed a WD 240GB SATA SSD in my HP notebook series 15 laptop as it was super slow in whatever it did. The HDD was in the first slot and the SSD was installed in the second(free) slot as I read that a SSD can help improve the performance when the OS is moved into it. I tried changing the boot order by pushing the SSD as the primary storage device and the HDD as the secondary one, but failed as it said "no bootable devices found" once I change the order and hence the HDD is again given priority. I want to know how I can change the order and also how do I move my OS into the SSD from the already present HDD??
If you don't have an OS on the SSD then yes you will get a no bootable device error since there is nothing on the SSD.
You can use a program like Macrium Reflect to clone the old drive to the new one assuming there is enough space on it. Best way is to make a boot disk for Macrium, boot off that and do the clone. Make sure you have backups of your data and be careful of what drive is getting cloned since you clone the blank new one over the existing one with files on it. Or make an image file of the existing drive to an external backup disk, then remove it, install the SSD and restore the image on it.