All kinds of reasons. If you are absolutely sure you have the right filesystem drivers built in, you can also try to change the root filesystem that is mounted on startup with the linux "root=" bootparam (man bootparam). "rdev" can be used to permanently change the root partition...
You can also get these very descriptive "target problem" errors if you confuse POSTROUTING with PREROUTING in the nat table when you're trying to setup NAT rules (e.g. adding a rule to POSTROUTING when you want to do DNAT or adding a rule to PREROUTING for SNAT respectively)