I guess I have cracked it!
Based on your last suggestion I tried to find the files which weren't being rendered properly based on their size but most of them were almost similar in size. There was still a need to find the hosts on which disk graphs were blank. I had a closer look at the associated .xml files and some of them had the following entries:
<RC>256</RC>
<TXT>update failed</TXT>
This gave a new direction and I did the following to find the hosts on which the graphs were blank:
cd /usr/local/nagios/share/perfdata
grep -i --color '<TXT>update failed</TXT>' $(find . -type f -name "Disk_Monitor.xml" )
It listed all the hostnames and files on which graphs were blank. I had a look here and there but could not find a way to repair the rrd files so the last choice was to remove them
for i in $(grep -il --color '<TXT>update failed</TXT>' $(find . -type f -name "Disk_Monitor.xml" ) | sort) ; do rm -v $i ; rm -v $(echo $i | sed -e 's/xml/rrd/'); done
Unfortunately all the old data is gone but the new graphs are being displayed now.
Thanks