When I bring up Gparted in Linux Mint it shows my hda1, hda2. If I had a second HDD would it be hdb or sda. I'm confused, what are all these designations? Thanks.

Classically, hdX referred to a PATA drive while sdX referred to a SCSI drive. Sata drives will show up as sdX.

Newer kernels now use one of the scsi kernel modules for PATA drives as well, so on a newer Linux installation, you may not see any hdX references at all. They weren't happy with some of the pata code and how well it was maintained.

USB drives use scsi code and show up as sdX devices. SCSI drivers are used to handle USB drives in MS Windows as well, since USB is a bus that is how USB drives are implemented, and coded internally, pretending that they are scsi drives.

On systems with PATA the standard is two IDE PRI-0 and SEC-1
(different manufacturers may call them a little different). Each IDE has two channels Master and Slave so in total you have 4 channels. Linux sees them like so.

hda = PRI - Master
hdb = PRI - Slave
hdc = SEC - Master
hdd = SEC - Slave

SATA = sdx
SCSI also is seen as sdx

And as jschiwal has stated with the newer kernel's hda is seen different, although you can still use the old standard when editing your fstab.

USB drives are also seen as sdx.