I'm running DPM 2019 on Server 2016 with modern storage. I have 4 x drives in the storage pool. One of them has exceeded the 90% limit but DPM seems incapable of using free space on the other drives. Can anyone tell me why?

The warning message is as follows:
Description: The used space on the DPM storage pool volume F:\ has exceeded the threshold value of 90%. If you do not expand the volume, backup jobs may fail. (ID 33514)
Recommended action: Expand the storage pool volume or move storage for some data sources to other storage pool volumes.