I have an EeePC with 2 internal SSD drives, a 4gb and a 8gb. When I try to install from the SD card it boots up on, the install screen says I only have 8.1gb and I need 8.6gb to install.
I’d like to manually split up the disks, maybe with boot, root, and swap on the 4gb and usr on the 8gb or something similar.
My question is, how can I get around the install screen check and manually slice the disks?
TIA,
Rick
Hi, welcome to the community! 
Ubiquity is the name of the installer. It seems others have requested to have the ability to skip the free space check, but sadly, hasn't been implemented. 
There is a workaround in comment #34:
Quit the installer to go to the LiveCD desktop. Press alt+f2 and type:
gksu pluma /usr/lib/ubiquity/plugins/ubi-prepare.py
Edit line 53 and change False to True:
self.controller.allow_go_forward(True)
Continue with your install. Of course you should create separate mount points for /home and /var if you have multiple disks (like on EEEPC) so that you don't run out of space and / has enough to run.
This was in 2015 for 14.04, but I assume this will be the same for 16.04.
The other option for a "disk to be seen as one" is to create a LVM, software RAID or hardware RAID (if supported and available in your BIOS settings)
A disadvantage of a RAID 0 is that if one of the drives fail, the entire installation is unusable.
Thanks for the quick reply. I’ll report back as soon as I get it all done, yes it is for 16.04.
Rick
Ok, that worked perfectly. The line to change is now line 57 on 16.04 .
Here is my layout and disk usage after the install and an update:
rick@ra:~$ df -h Filesystem Size Used Avail Use% Mounted on udev 986M 0 986M 0% /dev tmpfs 201M 6.4M 195M 4% /run /dev/sda1 2.7G 1.1G 1.5G 42% / /dev/sdb1 7.3G 3.2G 3.8G 46% /usr tmpfs 1003M 316K 1003M 1% /dev/shm tmpfs 5.0M 4.0K 5.0M 1% /run/lock tmpfs 1003M 0 1003M 0% /sys/fs/cgroup tmpfs 201M 36K 201M 1% /run/user/1000
Thanks again,
Rick