Non-icloud restoring of Photos stuck at 75%

Have a large non-iCloud Photos library on an external hard drive. Update to Catalina required reformat of Photos which took about 12 hours. Since then a backup to another external hard drive has not finished. I could still access Photos. Read in communities that Repair of Photos could solve problems. Tried that and got to Repair stuck at 100%. Photos could not be opened after that and went into Restoring... mode. Progressed slowly but overnight stuck at 75%. I could see both the Photos and Time Machine lights flashing. Turned off automatic backup in Time Machine. Photos Restoring... still at 75%.