Is there a reason you’re still on LR 6.0? I’d recommend upgrading to LR 6.1.1, not LR 6.2 or higher until LR 6.3 is available. Links can be found, here, although the scenario on the webpage is about rolling back from LR 6.2, the steps should work to get to 6.1.1:
As far as what’s actually happening to you, if a new catalog doesn’t help, then maybe your L: drive has issues and creating a catalog your internal C: drive would help diagnose this. If the photos, themselves, have issues, then moving the catalog won’t help.
A couple more unrelated things to try: turn off GPU Acceleration in LR / Edit / Prefs / Performance, and purge your Camera Raw cache in LR Preferences / File Handling.
After exhausting all other options I finally managed to fix it by first uninstalling Lightroom, Deleting the corrupt catalogue then completely removing all Lightroom keys from the entire registry. Rebooted my PC and reinstalled LR6 an now whilst it is importin the pictures again it finally loads cr2 files correctly, only a little slow now as it is still importin photographs (takes about 7 hours) but I am certain that the problem is fixed,.
Thank for your support