averaging other channels and then maybe inverting it?
Probably Cz = -(average all channels)
After I updated my Brainstorm, I actually get a slightly different error:
Fixed: Bugfix: Non-blocking history management errors (fix) · brainstorm-tools/brainstorm3@fa05a04 · GitHub
I also fixed the History field in the ERPLAB import, solving the problem at its root: Bugfix: Fix history field with ERPLAB import · brainstorm-tools/brainstorm3@6921584 · GitHub
And here is link for the DatMat file and the sample erp data.:
How did you end up with an imported file of 400Mb?
When you import a file that is already epoched, use the menu "Import MEG/EEG", not "Review raw file":
https://neuroimage.usc.edu/brainstorm/Tutorials/ChannelFile#Review_vs_Import