The 8 Gb of flash memory you find on a SSHD drive does not strictly work in the same manner as a pure SSD drive you buy.
It is as you said set up to be a cache.
The algorithm of the drive via the firmware simply takes note of the most frequently run programs of the Mechanical side of the drive and then stores it on the 8 gb of Nand flash for faster running and loading.
When you format the drive only the Mechanical side of it should be accessed to format it Vaz
Still if you did format the 8 Gb nand flash.
All it will mean is it will take a day or so before any frequently launched program from the Mechanical side of the hybrid drive to be stored on the 8Gb flash nand used for faster loading.
Once the algorithm works out for example to...