The Disk sensor does not return the proper values for partition size or amount
of space used:

For example, /dev/hda1 is mounted at /. According to df, here is the used/free
info for /dev/hda1:

Filesystem           1K-blocks      Used Available Use% Mounted on
/dev/hda1              5662700   4972504    690196  88% /

But the Disk sensor says that it is 87% free and cites: 607.60M/691.25M. That
partition is much larger than that.

For /root, df says:

/dev/hdb1             39516244  35083196   2425728  94% /root

But diskinfo says: 88% free: 4.18G/4.71G. That is a 40GB drive.

I think this is due to the fact that the Disk sensor assumes a partition block
size which not all users have.