new File( 'c:' ).canonicalPath

gistfile1.bat

C:\Users\evgeny>java -version
java version "1.6.0_27"
Java(TM) SE Runtime Environment (build 1.6.0_27-b07)
Java HotSpot(TM) Client VM (build 20.2-b06, mixed mode, sharing)

C:\Users\evgeny>groovy -version
Groovy Version: 1.8.3 JVM: 1.6.0_27

C:\Users\evgeny>groovy -e "print new File( 'c:' ).canonicalPath"
C:\Users\evgeny
C:\Users\evgeny>cd .gradle

C:\Users\evgeny\.gradle>groovy -e "print new File( 'c:' ).canonicalPath"
C:\Users\evgeny\.gradle
C:\Users\evgeny\.gradle>cd \

C:\>groovy -e "print new File( 'c:' ).canonicalPath"
C:\
C:\>d:

D:\>groovy -e "print new File( 'c:' ).canonicalPath"
C:\
D:\>cd Temp

D:\Temp>groovy -e "print new File( 'c:' ).canonicalPath"
C:\
D:\Temp>c:

C:\>groovy -e "print new File( 'c:' ).canonicalPath"
C:\
C:\>cd .m2

C:\.m2>groovy -e "print new File( 'c:' ).canonicalPath"
C:\.m2
C:\.m2>