Grahame,
The SMSP Logical Devices are not used for SharePoint Databases, Logs, or SnapInfo. The Logical Devices are used to store RBS/EBS BLOB and Index Data as well as the SMSP Backup Index. You should not define the Log, DB, and SnapInfo LUNs in the LUN Monitor.
Logical devices are managed by the SMSP Media Service and the LUN Monitor is used to monitor these devices not the DB, Log, or SnapInfo LUNs. (As a side note, the monitoring is not realtime, SMSP checks every 30 minutes for space on the monitored devices.)
Logical Devices map to Phyisical Devices. A Physical Device can be a LUN, a CIFS share, or a SnapLock Volume. Logical Devices are used to abstract the location of the data so you can change the physical devices as your environment changes.
SMSP needs a Backup Index to store backup plans, backup metadata, and streamed backup data from the WFE backps. The SMSP backup index is generally a LUN connected to the SMSP Management server. From the error message you received, it appears you do not have this defined.
Thanks,
Mark