Starting with R5 Lotus Domino is able to rebuild the database views much faster than this was the case with R4.
To be able to use the new rebuild mechanism Domino needs enough disk space in the temp directory.
If there is not enough space you see the following error in the log: "Warning: unable to use optimized view rebuild for Viewname due to insufficient disk space .... Using standard rebuild instead".
In this case you really should take care of the problem as the standard rebuild takes much more time. We currently measured some hours instead of 30 minutes when rebuilding the views of the Domino Directory or the ADMIN4.NSF after an R7 upgrade.
The amount of disk space needed was almost up to 10 GigaBytes which was not available on the system drive.
You can either delete some files if possible or you can move the directory where the indexer task writes its temporary files to another location by using the NOTES.INI variable "view_rebuild_dir=X:\pathname\". You have to restart the server after you change this variable.