DSM-G600, DNS-3xx and NSA-220 Hack Forum

I am using "DNS-323 Rsync Time Machine! by raid123" for my backup strategy.
everything has been working great but now i am getting error about my backup drive being full.

No space left on device (28)
rsync error: error in file IO (code 11) at receiver.c(298) [receiver=3.0.2]
io timeout after 30 seconds -- exiting
rsync error: timeout in data send/receive (code 30) at io.c(239) [sender=3.0.2]

The issue is I have 2 huge files which are true crypted. the files are each 150GB. the total size of my
my backup drive is 500GB with 89GB remaining. I have manually ran the backup script and see it trying to completely backup/copy one of the 150GB files to the backup drive. Know I thought rsync handles these as binary files and would only download the difference. So I am guessing this was a bad assumption.

Can someone who understand rsync please explain what I am seeing and if there is an atlernative commandline
option to use like "-append".  The goal here is to do one complete backup of the file once and only backup the deltas in the image.

In the mean time I will google it

Here is my commandline  options used
$ffppath/bin/rsync -aivx --link-dest=$dstpath/current .........

Thanks
Newbie2