tests for both, whether the data have been preloaded and fname. fname again is constructed some lines above, considering what is set in the parameter overwrite.

One would expect that overwrite is the only parameter that counts, and wonder why one still gets an error message. The error message should be either split, saying sth. like “When overwriting files, data need to be preloaded” or the current error message should add such information. Without this information, the user is puzzled (given that overwrite was set to True).

I suppose, the same applies to save in Epochs (and perhaps in other classes too).

Thanks!