I've got a pretty robust backup strategy going. One of the media I use is a USB flash drive which I keep in a safe, this one is used least often. I must have not backed up since the last daylight savings time, because I just went to run a backup to it and I noticed every single file was being replaced because of the modification time. Before it finished I checked the mod time on my computer compared to the usb flash drive and noticed everything was off by an hour.

How can I avoid these full backups twice a year?

Also, isn't rsync supposed to do differential transfers? If the file is the same except for the mod time? Shouldn't just the time be updated? My laptop is using ext3 and the usb flash drive is using vfat. If I change the flash drive to ext3, will anything change?

From rsync's man page:

The above tells you that rsync specifically looks at modification time to determine whether a file has been changed. You have to add the --size-only flag if you do NOT want it to look at modification time.

I want it to look at the modification time. My problem is two-fold.

1) Twice a year the modification times are going to change (or so it seems). This isn't so bad except for:

2) The files have been unmodified. Shouldn't the rsync algorithm not be transferring the whole file? I even have the --no-whole-file option explicitly set. Changing the modification times on every file shouldn't take all that long, except in my case it seems the whole file is being replaced.