I observed the issue that IPCop is unable to install on storage devices which hold non-IPCop partition data already during my intensive testing with the 1.9.x line, but it seems that this issue is still with actual release. I got this issue also reported from some friends who use IPCop; actually a friend repoteded it to me who wanted to use a HD which was formerly partitioned by M0n0wall.
In all cases it helped to wipe the partion data with something like:
dd if=/dev/zero of=/dev/[h|s]da bs=1k count=1000
but I wonder why the IPCop installer cant correctly partion itself or wipe the storage device 1st - although it claims to do so with its warning .... :-P
its always troublesomely to 1st do the wiping with a LiveCD - specially when the firewall hardware is an embedded system with a flash where you need to disassemble the whole hardware in order to pull the CF ...