Problems with Excel – Linking Workbooks and protecting formulas

I have a situation where we have two workbooks which populate a third work book. Our users tend to put data in the cells in the wrong location and accidently delete formulas. So I have gone through and locked and unlocked cells and ranges of cells. Then I protected the sheets. but now when i copy the spreadsheets to a new location and try to relink them (Change Source) I get an error message saying that the workbooks can’t be found or the data is encrypted. Any thoughts on how to get around this.