I’ve got a RPI2 and have found, much to my dismay, that a 32 GB SD card is not enough space for MATE 15.10.3. So I drag out a 2 TB USB drive and have it all hooked up nice and proper, ahem!
My question is how do I make it a continuation where the SD card leaves off? I’m not adverse to jacking with config files. I’m more into the spirit of thumb twiddling.
Stephen
I keep on getting running-out-of-memory messages but I downloaded Gparted and it shows I have 26.6 GBs of unallocated space. Why the “scare tactics” of running out of memory? Doesn’t ye ‘ole NOOBs’ way of expandable memory still apply or do I need to reformat my SD card without that box checked?
You have a 32 GB SD card, on which you have Ubuntu MATE installed. But it's installed on a partition, which is only 4 GB big. So, you need to resize that partition to fill the entire SD card.
From the instructions on Ubuntu MATE for RPI page:
Re-size file system
There are no utilities included for automatic file system re-sizing. However, it’s not hard to do manually. Once booted:sudo fdisk /dev/mmcblk0
Delete the second partition (d, 2), then re-create it using the defaults (n, p, 2, enter, enter), then write and exit (w). Reboot the system, then:
sudo resize2fs /dev/mmcblk0p2
BTW, I prefer just sticking SD card into another computer, then with GParted resize the partition on SD card to use all of the available space.
Try this first, then if this isn't enough disk space, we'll talk about your external 2TB drive 
Found THE ANSWER here: How to move a running MATE installation to a hard disk?
I was, in my insistent tinkering, able to open up the rest of the SD card. I made it my swap drive…
So now I have a lil’ boot sector on the SD card and a 26.06 GB swap sector. All my data resides on the 2 TB drive. Shweet!