localhost root # resize_reiserfs -s +5G /dev/cobd/2
resize_reiserfs 3.6.19 (2003 www.namesys.com)

/dev/cobd/2 is of 3932160 blocks size only with reiserfs of 3932160 blocks
size on it. You are trying to expand reiserfs up to 5242880 blocks size.
You probably forgot to expand your partition size.