vCD requires 200 MB of free disk space on whatever file system /opt/vmware/cloud-director lives in, and the init script will check that prerequisite every time the cell is started.
If you run this command, does it show at least 200 MB of space available? df -h /opt/vmware/cloud-director
e.g.:
df -h /opt/vmware/cloud-director
Filesystem Size Used Avail Use% Mounted on
/dev/mapper/VolGroup00-LogVol00
13G 6.8G 5.2G 57% /
Recall that if you've got a single cell, any vApps or media you upload will be stored in that file system, and the product logs can grow to as large as 400 MB (total).