Power fail in midst of large export

I picked a bad time to export every DNG shot with my Pentax *istDS into TIFF format. About a third of the way through the export, the power failed due to the first nor'easter of the year. I got about 2700 of 10K images exported. I am now seeking a way to complete the export without re-doing the entire job.

I thought I was having trouble with the large size of the export, so I divided the source DNG images into three collections, called "first", "second", and "third". Each collection contains around 3500 images. When I display a collection, select all, then export, I see a dialog called Problem Exporting Files which contains a scrollable list of the TIFFs already created. When I choose "SKIP", the dialog dismisses and I see a brief status bar called "Preparing to Export". Then NOTHING.

Am I on the correct path? Is 3500 files at a time still too many? Will I get further if I break down each collection into subcollections? I suppose I could remove the exported TIFFs from disk and start over if I must.

The power outage has really thrown this process off course!