The following code returns serial number for C: volume

DECLARE SHORT GetVolumeInformation IN kernel32;
	STRING lpRootPathName, INTEGER lpVolumeNameBuffer,;
	INTEGER nVolumeNameSize, INTEGER @lpVolumeSerialNumber,;
	INTEGER lpMaximumComponentLength, INTEGER lpFlags,;
	INTEGER lpFileSystemNameBuffer, INTEGER nFileSystemNameSize

cSerial = 0
IF GetVolumeInformation('c:\', 0,0, @cSerial, 0,0,0,0) <> 0
	? "Volume Serial Number:", cSerial
ENDIF

The GetVolumeInformation is declared in a simplified way in the code above.

Check complete version of this code sample:
Basic Volume Information
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