If you have a project in "/Volumes/disk_name" as per your example, then an absolute reference to a file would be:
"/Volumes/disk_name/data/myfile.csv"
The equivalent relative reference would be:
"data/myfile.csv"
If you move the project to e.g. a Windows PC, then you could still use the relative reference even if the project was moved to e.g. "C:\Documents\NewProject".