Open Disk Utility in your Applications/Utilities folder, select your boot drive, First Aid and click Repair Permissions.
A long list ensues, just ignore it and try what you were trying again.
"files cant be copied onto the backup disk because it appears to be "read only"
Normally read-only files can be copied without problems. The "it" in that message probably refers to the "backup disk" not "files", and might indicate a damaged TM disk. See this thread: