I was trying to export a file to a folder when I realized that it was on an external hard drive. So I attached the hard drive with the export dialog box open and LR froze. It had to be killed via Task Manager. Latest LR 7.5 Desktop version. Windows 10 with latest updates 16 GB RAM
" when I realized that it was on an external hard drive"
Which was on the External Hard Drive? The file you were exporting? The folder you were exporting 2? When you reattached the hard drive, did Windows assign it the same drive letter that your Lightroom catalog was expecting?