The multiDiskTable OIDs from CHECKPOINT-MIB are missing in R81.20. They are available in the R81.20 mib, but the gateway responds that "no Such Instance currently exists at this OID".
The alternative is to use hrStorageTable from HOST-RESOURCES-MIB to monitor disk space. This is not great, since values are allocation units and space has to be calculated, which results in grater complexity and inaccuracy.
If anyone knows of any other alternatives or if the original table will be fixed in R81.20, please let us know.