You might not have to reformat - this sounds as though it could be an
owner/permissions problem to me. Open up a terminal window and type
'sudo nautilus', entering your password when prompted, you can then
navigate to where you have the files stored, right-click on one of them
and select 'Properties'.

Under the permissions tab, you'll see the owner, group and 'other'
permissions. Chances are that you'll be subject to the 'other'
permissions which, I'm prepared to bet, will be 'Read only'.

As you've 'sudo'ed' into nautilus, you'll have root access and so can
change owner, group and permissions to how you want them - or simply
delete the files, if that's what you want.