Thanks for reaching out. We are sorry for the trouble with Lightroom Classic. We are here to help.
Where is the catalog stored? Is it on the internal drive or an external/network drive? Where are the folders stored? Are they on the internal drive or the external drive? Does it happen with smaller batches of images too?
You are on macOS. You can ensure Lightroom Classic has the permissions needed on the mac by following the steps suggested below:
Go to the System Preferences for your mac, then Security and Privacy, then select the Privacy tab.
Then select Accessibility from the list to the left & check if Lightroom Classic is added to the list with a checkmark in front of it.
If Lightroom Classic is not a part of the list, please add it by unlocking the window first using the lock at the bottom left, then clicking the + icon & navigating to the Applications folder.
Also, grant permissions to Lightroom Classic under Full disk access found in the same list as Accessibility.
Once done, restart the mac.
You can also run a 'First-Aid' diagnosis on the external drives using the Disk Utility on the macOS and then restart the mac.