The new drive won't work internally until it's properly partitioned and formatted for use on Mac.
Does the new drive show up in Disk Utility when connected internally?
If it does, is it grayed out? If the new drive appears in Disk Utility but appears to be unmounted, you can select it and click the Mount button to manually mount it. Besides, check its partition map. If it's formatted with Master Boot Record, reformat it with GUID Partition Map.
Follow the steps below to format the drive for Mac:
- Open Disk Utility from the Applications > Utilities folder.
- Select this drive from the left sidebar and click "Erase."
- Set the Scheme to GUID Partition Map. (Very important !!)
- Choose a proper file system. Select APFS if it's going to be used on macOS 10.13 or later. Otherwise, select Mac OS Extended (Journaled).
- Give the drive a name like Macintosh HD.
- Click Erase again.
If your drive doesn't show up in Disk Utility, it's recommended to reformat it after placing it in the USB enclosure and connect it to your Mac externally. Use the same steps mentioned above. Once the new drive is successfully formatted, you can reattach it to your Mac internally and check if it works now.
If that doesn't help, grab another 2.5-inch and 3.5-inch drive and check if your Mac recognizes them when connected internally.
If your Mac recognizes the 3.5-inch but not the 2.5-inch, the problem likely lies on your 2.5 to 3.5" adapter. In this case, change to a different adapter.
If your Mac doesn't recognize both of the drives or both drives, you need to replace the SATA hard drive cable.