using "-f" option on windows

I have global version  5.7.4. Until now it worked great. However now I have
read-only disk with sources to index. Situation:
Read-only disk is "Z:". Being on "Z:" I do following:

gtags -f d:\workspace\files.txt d:\workspace

for each file listed in "files.txt" I got message:
"Warning: '...path here...' is out of source tree. (Ignored)"

I have tried several variants of path in "files.txt"
z:\dir1\dir2\file.cpp
z:/dir1/dir2/file.cpp
dir1/dir2/file.cpp
dir1\dir2\file.cpp

and always got "out of source tree". Is there any way of indexing this
files?
Is it a bug or rather I am doing something wrong?

regards
Mikolaj
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