This is a QTS design limitation. When QTS is initiated, it sets a SWAP partition in the Turbo NAS HDD. It means that removing a HDD will cause unpredictable system behavior, including freezing and crashes.
If your Turbo NAS is set to RAID 1, RAID 5 or higher, it can tolerate the removal of a single HDD. But if your Turbo NAS only has a single HDD or is set to RAID 0 or JBOD, you will encounter system freezing or crashes and the only way to recover is by reinserting the HDD and restarting the Turbo NAS.