Maybe the "timeout" problem is OS specific. XP Home regardless which service pack is not a network friendly OS. It wasn't designed for extended networking or things like that.
XP home is rather primitive in it's networking capabilities but i can't tell if MS updated it with services packs.
XP locks / keeps the folder for access rather long time so you encounter problems by beeing faster than the OS like it.
The 6 hour locking is rather odd and my be influenced by the crossover connection. Too bad you have no alternative to set up other networking method.
Try again but deactivate second LAN card in windows after you powered off the PS2 and check if the files are still locked, if not we can say it's windows that still providing / keeping access to the files for the PS2 if you don't deactivate the LAN card.
Try it and report differences so we can pinpoint more to the problem in windows.
PS2 SCPH-70004 V12 PAL + HDCOMBO + 160 GB Samsung HDD + OPL 0.9.1
PS2 SCPH-70004a V13 PAL Satin Silver
2 x Official Sony 8mb card +FMCB 1.8c + uLE 4.42b
Datel Max Memory 32mb card +FMCB 1.8b + uLE 4.42
Memor32 + Memento FW 1.2e