Well, actually this seems to be related, does normal RTW show the same failure if starting a new campaign?

My gray cells might trick me, but I think to remember a unsolved problem with RTW and external drives...

A thing worth a try would be to install EB on your normal computer, start it and load up a campaign, afterwards move the whole RTW folder to your external drive.
It also might work if you start a campaign on your PC and copy the save-file to the external drive and load it up there.

XSamatan