dfu_upload error -84 when trying to backup images

[shawnzier at gmail.com]

I'm getting the following error when trying to backup rootfs:

dfu_upload error -84

I've been able to back up the other partitions, but the rootfs fails
every time at 258076672 Bytes. I think this is the size of the rootfs,
but how can I be sure?

I don't want to flash it with this image if there is missing data. I
also don't want to try to flash it with 2008.8 if I'm not sure that it
will complete. If it doesn't complete, will I be left with a brick until
I can get it to complete successfully, or does it not write until the
whole image is transferred?

The host is a Macbook Pro CoreDuo (not C2D) running Gentoo.

Has anybody had a similar experience?