One of the easiest steps overlooked is setting the jumpers! I've made this mistake more times than I can count and know how frustrating this can be. If you have both drives on the same IDE channel, ensure the jumper settings determine the new drive as Master and your old drive as Slave. In some systems, however, you may need to set them to cable select. Another option is to have them both mastered (or single) on separate IDE channels, depending on what you already have on the particular channels. If your drive doesn't have jumper lables, ie; MA | SL | CS, Samsung's website should have thd documentation.
Regardless of the jumper settings, the BIOS will see both drives if they are plugged in properly.
Another thing may be filesystem. If your primary drive is, say, FAT32 and your secondary drive is NTFS, the FAT32 will not recognize NTFS.
Hard drives are sensitive just like most other computer parts. One thing, as much as I'd hate to say it, if the jumper settings and location on the IDE cable make no difference, it may have crashed in the change. Sometimes it just takes a little jolt to cause the read/write head to make contact with the platters inside the drive.
Hopefully, that is not the case and it's a simple matter of jumper settings or location in the chain.
Good luck! I'd be interested to know what the resolution is.
Regards,
Jesse