I tried Paul Reed’s script for moving the operating system to a USB HDD. It does just that but the (third) Data partitition remains on the SD card.
Inspired by this I have managed to move both OS & data partitions to a USB HDD.
My ‘build notes’ are as follows and they maybe helpful …
JB_usb_hdd.txt (2.8 KB)
Designating the USB HDD as sdaX (rather than using UUID’s) is not preferred but it is most unlikely that more than one will be used which can result in drive mis-identification