Assuming that your check_nt_diskspace command maps to check_nt -v=USEDDISKSPACE, according to check_nt --help this is...
Size and percentage of disk use.
Request a -l parameter containing the drive letter only.
Warning and critical thresholds can be specified with -w and -c.[/blockquote]
...perhaps as it references both "Size and percentage" then you need to include a % character in the warning and critical thresholds, like
No way of knowing for sure though without seeing the object definition for check_nt_diskspace