I have not tried it myself with this size (30Tb), but according to information from Fantec, the limit is open at the top. I have 4 x 18Tb in RAID 5 running without any problems.

Answering my own question after email exchange with the manufacturer. It’s not possible to add a HDD to a raid 5 array with 3 disks. Raid array needs to be inicialized from scratch

This should work, these RAID levels are supported: RAID modes 0/1/3/5/10 & BIG However, it should be noted that this is only a hard drive enclosure and not a NAS server. But the price is quite reasonable.

The question does not arise in itself, because the device is built as a hardware raid, thus independent of drivers, and can therefore be connected and operated with any file system and thus with any operating system after raid determination and raid hard disk synchronisation (which can take a long time depending on the size of the hard disks used).

No matter which hard drive is attached to the Raspberry, without knowing when the hard drive(s) should go into sleep mode, they will continue to run. Since Raspbian does not support this by default, you have to install the function. There are programs like "hdparm" or "hd-idle" which you have to use (I work with hd-idle because my Seagate HDD is not compatible with hdparm). Just start Google and search for hdparm and hd-idle.

Yes, this model (look for the R in QB-35US3R) has a proper RAID controller built in. RAID 0, 1, 3, 5 and 10 can be configured on the enclosure.