Trouble with Time Machine?
If you have a new external drive that Time Machine won’t let you use for backups, make sure that it is formatted correctly. Time Machine can only write to disks if they are formatted as ‘Mac OS Extended (Journaled)’.
Most USB drives are sold preformatted with the Windows NTFS format, so you’ll need to reformat them using Disk Utility. You also can’t have a slash character in the volume name. Disk Utility won’t let you create a volume with a slash in the name but if you have rename the volume using Finder, it will allow slash characters to slip past.
Sometimes Time Machine will fail with the message “An error occurred while copying files”. The first time this occurs, just run the backup again manually; it may succeed this time if the error was caused by a transient USB glitch. If it fails twice, use the Time Machine Buddy dashboard widget (free, bluedog.com.au) to see which file caused the problem. Try excluding this file in Time Machine’s preferences. If the backup then works, this file may be corrupt. Try deleting it and recreating it from scratch. As a last resort, turn Time Machine off, delete /Library/ Preferences/com.apple.Time Machine.plist. Then turn Time Machine back on again.