try gdb --args ./pg_upgrade -d /var/pgsql-8_4_3/data/ -D/var/pgsql-9_0_1/data/ -b /var/pgsql-8_4_3/bin/ -B/var/pgsql-9_0_1/bin/ --check -P 5433 -v -g -G debugand when it fails, type in 'bt' and paste it here please.
-- GJ