I got the error message below when trying to use the Portraiture 3 plugin inside LR Classic CC. I use a WD external USB drive where photos are located. I've checked permissions to write/read and it seems ok. I just can't solve this annoying issue.
'Lightroom was unable to prepare the selected file at /Volumes/WD Ultra/BackupMinhasFotosLR/2017/2017 set 17/pricila-30.dng for editing.