Thanks for your case it helped us to realize what enhancements we need to include in the next update. Since there is a VSS limitation regarding FAT volumes – the low free space for snapshot warning seems to be obsolete.
Meanwhile, I believe the source NTFS volumes still should have a low free space warning during backup process because when there is no free space for a snapshot backup will simply fail. Found the some highlights from MS KB (Troubleshoot VSS issues...
“There is not enough disk space to create the volume shadow copy on the storage location. Make sure that, for all volumes to be backup up, the minimum required disk space for shadow copy creation is available. This applies to both the backup storage destination and volumes included in the backup. If this occurs, besure that the following minimum requirements are met:
For volumes less than 500 megabytes, the minimum is 50 megabytes of free space. For volumes more than 500 megabytes, the minimum is 320 megabytes of free space. It is recommended that least 1 gigabyte of free disk space on each volume if the volume size is more than 1 gigabyte.”