Now boot into the Zorin OS USB stick (it's always best to reboot after you do any heavy disk operations, just to be sure that everything's working OK), select "Try Zorin", let the machine settle down, then click the icon on the desktop to "Install Zorin".
Don't just click "Install Zorin" right as it boots... sometimes all the drivers aren't properly loaded at that time, and the install takes its cues from what's running in the "Try Zorin" mode. When I did "Install Zorin" right as it booted, it didn't go well for me, but selecting "Try Zorin", waiting until the machine was fully booted and settled down at the desktop, then clicking the "Install Zorin" icon on the desktop worked just fine.
You don't have to partition or format the drive before installing... the installation will take care of all that, and the format you just did on all the partitions of the internal drive should have forced the drive to remap any bad sectors... so you should be good to go.
If the installation wants to re-partition the drive, it's likely it wasn't aligned along 4K sectors (which may be why it enumerated so slowly)... if so, let it re-partition the drive. It generally leaves a bit of unpartitioned space at the beginning of the drive to accomplish this alignment.
Then, after you've got the install done and a couple of reboots performed to be sure it's working alright, go back to the top of this post and remove the startup items that slow down the boot (Switcheroo and all that).
