Thanks jeffreybrown. I can't do much with the file as it won't even open, presumably as it is so large. I use it daily to log data and as I have found an earlier version I could revert to that, but that would mean losing about 10 days of data. Consequently I thought if I could open the big file in another application, such as notepad, I might be able to copy the missing data and then just delete the big file, but Notepad can't do this and I don't think a suitable app exists. Would I just be able to change the extension on the file name to .xlsb?