I got a 4.5 TB hard drive not too long ago to back up all my data on. I've been backing it up regularly without too much incident. Typically, what it does is back up everything new and everything that has been modified; that way, it doesn't have to copy EVERYTHING a second time. However, this most recent time, it tried to back up EVERY SINGLE THING I HAD, including almost everything that was already on the drive, which took ages (ironically, when it was done, it didn't even have everything it was trying to back up; some of the files disappeared from the drive, including the ones I was most concerned about backing up). This also means that the copies of the old files take up an extreme amount of space (probably about 250 GB). I deleted all of that and tried it again, hoping it was just a fluke and that it would go back to normal if I simply tried it again, but it just started backing up EVERYTHING once again. How do I fix this?

Perhaps you've missed clicking an option as to which directories when you start the backup. Or you've unclicked an option by mistake that was normally there during past backups?

Been so long since I've done mine - it's a good reminder.

No, I didn't change any of the settings. I just clicked "Go", like I always do. I also didn't change the drive designation (i.e. I was backing up from G:, and I'm still backing up from G:), so that's not it, either.

Not sure I can help, but what software are you using, in case that matters to diagnose the problem?

I am, of course, assuming you are using a specific program to run the backup.

I have a Seagate Backup Plus Drive, so I'm using the Seagate Dashboard 2.0 that came with it.

Presumably, the Seagate program would have to determine the files to be backed up by looking at the files' Properties. This is where the created date/times and the Modified date/times are recorded. (Right-click > Properties > Details)

So the date and time of the computer clock might be worth a look at. Files will have date/time written to their 'Properties' according to that time setting. If this is wrong or has been changed, I think this could confuse the Seagate program's operation. Just a thought.

Going off of what tw said, the time change for daylight savings time, maybe?

Might not be a bad idea to scan through the forums on the Seagate site itself (maybe even registering and asking there). I don't have any experience with external backup drives and their software.

The forums are here: http://forums.seagate.com/