Hi Kirk
I quit and restarted PM+ last night. Then I ran a full sync with the same drive offline as before and left it to see where it was at in the morning. This morning the sync was stalled as before and at the same file as before. I stopped the sync and saved the log.
I searched for the file that PM+ had stopped at and found 2 in the catalog. One was fine, the other would not open and gave an 'offline' error message although it had a green dot in the corner. The path showed it as amdoc://[long string of characters][filename with spaces shown as %20]. I couldn't find it in a Finder search or delete it from the catalog but 'remove from catalog' worked. I think it was ghost of some kind.
Having removed that file I again ran a full sync. The window showed that it was beginning a search for missing files but no list of files appeared and the window remained blank after the initial 'Begin Sync for 1 catalog' message.
Activity monitor showed around 100% CPU. I left PM+ to itself, not doing anything with it but, after 45 minutes, nothing had changed and I needed to use it so opened a contact sheet. The sync window changed from white to black and the message 'scanning for missing files: 99%' appeared at the top.
An hour later nothing had changed so I stopped it, generated a new log, quit & restarted. I mounted the missing drive, tried again. Window opens, no scrolling list as it looked for missing files but % at the top increased at a reasonable speed and continued to an apparently successful conclusion.
The sync window is showing Files Added: 25226, Metadata Updated: 24957. Those are very large numbers, far higher than the number of files that I've knowingly added or updated and the files shown are largely ones I've not looked at for a long time. The sync collection also shows 25226 new files but opening that collection shows only 1456. This difference between the number shown as being in a collection and the number of files that appear when it's opened is a frequent occurrence, mostly with sync collections.
Unlike in your test, I'm consistently failing to do a full sync with this drive offline.
The logs are attached.
I see that there's a new version of PM+ but for consistency I've not yet installed it.
David