I recently installed Fedora 26 on my laptop: Lenovo Legion Y520, in the HDD (the laptop has 256G ssd +1T hdd). When I use Fedora, the temperature of my SSD rise high(70c+, much higher than when using Windows 10) I can notice this by just touching the part of the laptop where it is installed(the ssd is in the corner, far away from other hot components). The SSD is not even mounted.
Just because a drive is not mounted doesn't mean that the system ignores it. It's constantly polling hardware to see if devices are removed, added, etc. You don't technically ask a question, but I'm assuming it's 'why', and that is your answer -- the system still acknowledges a drive in the machine and wants to know what's there, and gets stats from it, and as I said before, polls it for changes often.
Normally, an unused ssd should not be hot, nor even warm. Perhaps it is doing some background housekeeping, but I doubt that this would last for long.
You might try asking Linux to remove the scsi device from its list.
lsscsi to find the host:bus:target:lun of the device, check it is not used with
lsof, and then write the number 1 into the appropriate sysfs device. Eg:
$ lsscsi ... [4:0:0:0] disk ... /dev/sdb $ sudo lsof /dev/sdb $ sudo sh -c 'echo 1 > /sys/bus/scsi/drivers/sd/4:0:0:0/delete'