I have since found out a little more. The NRRD image is approx 20GB. I am running on a system with over 60GB. On a previous read attempt, error messages regarding memory appeared. In this case other hanging processes from image conversion had used up the memory. With those processes terminated, Slicer successfully loads the file into memory but takes a long time about it.
This discussion can be closed. Seems like there might have been some issue with the TIFF volume not matching format. Conversion to NRRD has worked.
I think the dataset can probably be reduced. Bit depth if nothing else.
I came across the discussion here in 2024 that was helpful.