Hi, I`m fully booting from the Esata drive and I don`t get what you are trying to do !!
Do you want to boot from the SD card, but mount parts of the root to the Esata disk ?
Or just put a web site on the Esata drive and logs etc on the disk ?
MRTG isn`t that the old version of rrd tools , which I use plus I also use Monitorix , which gives me my system graphs