I have a XC95144XL that can not be erased, or when trying to program I get:
xc3sprog -c matrix_creator -p 1 534ram.jed
XC3SPROG (c) 2004-2011 xc3sprog project $Rev: 774 $ OS: Linux
Free software: If you contribute nothing, expect nothing!
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Erase still running 00
failed
Mismatch at fuse 0: 1 vs 0
I can see from fora that you need the Xilinx ISE Impact to set the erase flag on the chip https://www.exxoshost.co.uk/forum/viewtopic.php?t=3044
Would be great if xc3sprog could override to erase the chip ;-)