I don't use Mint, so I don't know if it auto-mounts the "other" hard drive ( non-OS drive ) automatically or not.
How to fix.
Open a terminal.
sudo fdisk -l
This will show you this disks you have. Likely /dev/sda and /dev/sdb is my guess.
It will show you the sizes of them. Probably the HDD drive is sdb.
Assuming this drive is sdb...
fdisk /dev/sdb
press "g" to create a gpt partition table.
press "w" to write this partition to the disk.
This will end fdisk.
fdisk /dev/sdb ( again )
press "n" to create a new partition.
Just press enter to accept the default values a few times.
Once the partition is created,
press "w" to write this partition to the disk.
This new partition will be called /dev/sdb1 ( the first partition on drive /dev/sdb )
This will probably kick you out of fdisk again, if not, press "q" to quit.
You can format this disk now.
mkfs.xfs or mkfs.ext4 or whatever filesystem you like. Likely ext4 is what Mint uses by default.
The syntax for this is...
mkfs.ext4 /dev/sdb1 ( don't forget the 1 )
When this is done.
blkid /dev/sdb1
You will see something like...
/dev/sdb1: UUID="a48485c1-57ae-49be-9b84-e89f8d9a34dd" BLOCK_SIZE="4096" TYPE="ext4"
Obviously the UUID string will be different.
edit your /etc/fstab file
add this line at the bottom
UUID=a48485c1-57ae-49be-9b84-e89f8d9a34dd /home ext4 defaults 0 0
(use your UUID, not the one I put here as an example )
When done editing this.
mount /home
df -h
You should now see /home as your 900GB partition.