Reece,
At first it is a bunch of "mumbo jumbo"... but it is very simple... once you know.
First the older stuff, since you remember DOS:
On older IDE systems the Hard Drives where listed as:
hda
hdb
hdc
hdd
"hard disk a/b/c/d"
The IDE controller had a "Primary" and a "Secondary" controler.
Each controller had a "master" and a "slave" drive.
If we made ONE PARTITION per drive,
--
DOS/WIN would see it as:
Primary Master: C:\
Primary Slave: D:\
Secondary Master: E:\
Secondary Slave: F:\
--
Linux would see it as:
Primary Master: hda1
Primary Slave: hdb1
Secondary Master: hdc1
Secondary Slave: hdd1
--
I think (it has been a while) the 3rd "letter" (hdA, hdB) is based on which controller, and whether it is the master/slave.
I'll cover partitions in the next section. Although you may have already guessed.
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Now let's switch to SATA drives.
SATA drives change the drive designation in Linux from "h" to "s".
My mother board has six (6) SATA connectors. I have four (4) 2TB drives.
These Hard Drives are listed as:
sda
sdb
sdc
sdd
I forget what the "s" actually stands for, but I always think "SATA".
The "Number" is for the partition.
(I have 4 hard drives in a raid0, striped RAID for speed),
For my first "Physical Hard Disk" my "drives" (partitions) are listed as:
sda1
sda2
sda3
sda4
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The upside of this "difference of opinion" is that when Windows automatically assigns a drive letter, it starts with the FIRST partition on the FIRST hard drive and labels that partition as drive "C:\".
Where as Linux would label it "hda1" for an IDE drive, or "sda1" for a SATA drive.
So looking at your screenshot,
sdb1, and sdb2 would be your 2nd hard disk.
IF in Win10, your 2nd Physical Hard Drive was drive E:, then "sdb1", is your drive E:
Since you selected "Install along Side Windows 10", Ubuntu's installer is NOT touching your system (1st) drive, BECAUSE YOU HAVE A 2ND DRIVE.
It IS using your 2nd Hard Drive because it does NOT have as OS on it.
To use Drive "D:\", you would need to install to SDA2.
Which means you will most likely have to do that manually.
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Let me know if I need to clear any of this up. I tried to "cover the basics" so you could understand any situations you run across, and also to address your specific situation.
It is a bit confusing at the beginning. But once you start thinking of your drives as hda/b/etc, you know EXACTLY which drive you are using/talking about!
Barracuda
P.S. ext4 is a Linux file system format. Like Fat, Fat32, and NTFS, it is the newest of the Linux File System formats. There are others BTRFS, etc....