Sorry that Lightroom Classic is not showing previews if images while importing.
Which exact version of Lightroom are you using and are you importing directly from the Camera or card?
It sounds like the images are being corrupted. Is there a difference between the corrupted images whether you are importing directly from the camera inside of Lightroom vs copying using finder and importing from the hard drive?
Does it help if you optimize the Lightroom catalog from the File> Optimize catalog menu?
Do you have an external device connected to your computer where some of these images are stored? Just to ensure, are you using the version 10.2 of Lightroom Classic? Check under the Help> System Info menu in Lightroom Classic app.