For some reason, this has happened twice now, I use a 500GB external USB2 hard drive for time machine back up, and both times, the time machine backs up SOOOO slow for no apparent reason. If I were to format the drive and redo the backup, the speed would resume to normal. Work for about 10-20 back ups then this problem occurs again. Time machine doesn't not show any errors, it just is slow. It takes about 2 hours for "Preparing Backup" then another 6 to backup 140 MB worth of files. At first I thought the drive might be bad but transferring a file via finder disproved that theory. Disk utility check was good also except the check was extremely long compared to before. There's no errors in console either.. its just slow.