Here’s a little puzzler based on a personal experience I had a recently. I had finally outgrown my backup drives, so I purchased two identical new ones online. When they arrived, I set about backing up my home server to both drives. I use Beyond Compare to make backups; while this is a wonderful file synchronization tool, in this case both drives were blank, so it was really just copying the files.

I unwrapped the first drive and began the backup. Then I unwrapped the second drive, but before I finished my cat came by with an urgent request, so it was maybe ten minutes later when I finished petting her and began the second backup. Also, on the second backup I forgot to exclude the recycle bin, so the second backup had several additional files compared with the first.

So of course I expected the first backup, with its significant head start, to finish first. Yet when I checked back in an hour or so, I was surprised to see the two backups were perfectly in sync! That is to say, they were copying the exact same files at the exact same rate, and ultimately they finished within a second of one another. Here’s a screen shot to show you what I mean:

As you can see, both backups are exactly in sync.

THE QUESTION: Why did this happen?

HINT: It’s not a coincidence; I can reproduce it at will. It’s also related indirectly to the theme of my previous post!