[dnanian]

In this case, it looks like you're getting an I/O error either reading or writing the file, which might be caused by a surface error on the drive.

Let's try re-creating the index. In Terminal, do the following:

sudo mdutil -i off / /SetiAlphaFive

Then, do the backup (at this point, your indexing is off for both drives). When done, do:

sudo mdutil -i on /

That'll turn indexing back on for the boot drive (and it'll recreate your index, too).