core4 partition permissions

I've got 2 hardrives,
hda has core3, Xp and a shared vfat data partition.
hdb has core4 and a vfat data patition.
when in core3 user has full write access to all partitions, but in core4 I can't get user write permision for the hda vfat data partition, although it will write to core3.
I've tried as root to change permisions or owner, but just says I don't have permision to do so.
I've tried disabling SElinux, but that doesn't help.
Any body got any other suggestions/ideas on what's going on???
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