You might try inserting a file from the new location into a container field with the "by reference" option, then examine the file path in the container field to see if it matches what your are trying to use.
Getvalue ( container field ; ValueCount ( container field ) )
will give you the path if you insert by reference.
At a guess, you seem to be missing either a drive letter or volume name.
A container reference has two file paths:
GetValue(FileMaker Reference Container ; 1 )
GetValue(FileMaker Reference Container ; 2 )
Create two calc fields, one for each value above.
You'll see the drive/etc designations for your reference.
Sometimes you have to reinsert the files after moving them...