Experimenting with Lion,
newfs_hfs and smaller journal sizes it appears that 512k is too small for its journal size because kernel hangs occur.
Google results show kernel panics for journal sizes of up to 8M:
panic(cpu 0 caller 0x001023CC): jnl: transaction too big (8385024 >= 8388096 bytes, bufsize 4096, tr 0x361cf7c bp 0x476bf4f0)
For now the 512k journal is disabled to prevent further panics.
What is the real world minimum HFS+ journal size for a 4.0TB hard disk drive?
And is there a way to increase the journal size without having to copy/move/clone all data?