Why is the project unable to find a file that is right beneath its nose?

Why is the project unable to find a file that is right beneath its nose?

I'm trying to build a solution that has three VB projects; one that I'm
working on, and two others that have interdependencies (although one says
"unavailable").
In trying to compile, I get regarding the ancillary project, "Unable to
find source file 'J:\DSDPAGE_T.xsl' for file 'DSDPAGE_T.xsl', located in
'[TARGETDIR]', the file may be absent or locked."
J? I searched the solution to see if "J:" is hardcoded anywhere, and it's
not, so...?!?
The "missing" files are right there n the project! Why is it looking in J?
If that is hardcoded somewhere, why doesn't 2-clicking the err msg take me
to the spot? When I do, a "File System " tab appears with two panes. On
the left:
File System on Target Machine
Application Folder
System Folder
User's Desktop
User's Programs Menu
...and on the right pane, a listing of the files in that project (the same
ones I see in Solution Explorer), many of whom are the subject of err msgs
that they cannot be found...?!?
Is this a "VB thing" (I've never worked with VB before) or...?!?
Maybe I can create a J drive and put those files the compiler is wearing
blinders about in there just so it will compile - it's not that project I
need to work on anyway, I just need to let the solution compile, so I can
update the other project.