Identified by adamsmith as a zotfile error - please see Zotero Forum Discussion item https://forums.zotero.org/discussion/63547/zotfile-tablet-transfer-error#latest

Essentially I have 4000+ items in my library that I wish to send as pdfs to a new HDD folder - call it C:\Zot_temp. Most, but not all, of these items have previously been sent to a different HDD folder (e.g. C:\Zot_pdfs) after retrieving metadata from pdf. When I now select the items & use "Mange Attachments" -> "Send to Tablet" I get the error msg captured on https://www.dropbox.com/s/zsypyg9zmw7sp1u/Zot1.docx?dl=0 (see towards end, and also most recent comments by adamsmith on above Zotero Discussion thread). No pdfs get transferred over.

Using latest Windows 10 updates & Zotero 4.0.29.10 with zotfile 4.2.7 (updated 13.11.2016).

Many thanks Richard