[KevinH]

A few thoughts:

Sigil can do spellchecking and find and replace across files without any need to merge things first. That said, this issue generally comes when a file with an ".xml" extension is not properly identified by media type. So here are some things to look at:

- Are you sure you have used ".html" as the file extension? Perhaps an embedded character or space got involved somehow? You could try renaming the file to be sure.

- How does the current OPF (content.opf) show that one file in its manifest? What media-type and exact name does it have?

- Are there any other files in the manifest that are ".xml" and NOT .html or .xhtml? If so what media-type do they have?

- If you do have ".xml" files, try removing them **after** saving them to files first so you do not lose anything, and try splitting again.