Remove music files from disk, but want to remain in library & devi

Running WMP11 on an XP box with small hard drive.

Got a Sony Walkman - been ripping in lossless format and downloading same to
device. I want to remove the rather big set of WMA files (from my box) after
loading to Walkman to conserve disk space. But, I want to keep albums visible
in WMP library and not have songs removed from the Walkman.

Don't want to invest in bigger, or external, hard drive as I'm getting a new
PC soon.

How do I "fool", or configure, WMP so it doesn't remove corresponding files
from my walkman after I delete same on hard drive (which it tends to do when
I sync up)?

As an FYI, I was able to do this successfully with four albums until I
updated "monitor folders" and it did a fresh update/sync - at which time it
deleted the 4 albums from my walkman.