Saving keynote file on external hard disk

For the past few days I encounter this. The saved keynote file on the external hard disk is opening fine, but I can’t make any change to any slide whatsoever, if I do, I can’t save it again. The message says the file is locked whereas it isn’t, then it asks me to revert the changes and am forced to oblige. At times it says the generic volume doesn’t give me authority to make changes to the files stored in it. Very perplexing, please help.