add support for external, removable drive backup

- select backup source by drive label, device name, or serial number
- fix forget policy to be safe for multiple drives (group by host,tags)
- tag each backup source with drive/source name

config.ps1

@@ -12,7 +12,7 @@ $GlobalRetryAttempts = 4
 
 # maintenance configuration
 $SnapshotMaintenanceEnabled = $true
-$SnapshotRetentionPolicy = @("--group-by", "host", "--keep-daily", "30", "--keep-weekly", "52", "--keep-monthly", "24", "--keep-yearly", "10")
+$SnapshotRetentionPolicy = @("--group-by", "host,tags", "--keep-daily", "30", "--keep-weekly", "52", "--keep-monthly", "24", "--keep-yearly", "10")
 $SnapshotPrunePolicy = @("--max-unused", "1%")
 $SnapshotMaintenanceInterval = 7
 $SnapshotMaintenanceDays = 30
@@ -30,3 +30,6 @@ $BackupSources["C:\"] = @(
 #$BackupSources["D:\"] = @(
 #    'Software'
 #)
+#$BackupSources["DRIVE_LABEL_NAME_OR_SERIAL_NUMBER"] = @(
+#    'FolderName'
+#)