Cannot edit photos on external hard drive using LR Classic

Dear All,

I cannot edit photographs that I have stored on an external hard drive by using LR Classic. LR will show me the embedded preview of the pictures in the "Library" module but when I switch to the "Edit" module, it tells me that it cannot find the file. In addition, I cannot move pictures from the external drive to my Macbook (or vice versa) using LR. The problem occurred recently (a couple of weeks / months ago - not quite sure when). Before that time LR worked well. I can open and edit the pictures on the external drive with PS. 

I have a 2021 MacBook Pro with an M1 chip and macOS Monterey Version 12.4. I have subscribed to Adobe CC and my versions of PS and LR Classic are current. The external hard drive is a 4TB LACIE Desktop Drive.