The X-MASS is an ATA/IDE interface for the CPC made by ToTO. The device uses the same IDE I/O ports than the Symbiface 2 and is therefore compatible with it.
The device also has a Disk-On-Module IDE device connected. It is possible to remove this and use other compatible ATA devices. The X-MASS has a single device which is the master. It doesn't directly support a slave device unless an adaptor is used.
When the X-MASS is powered on the CPC is held in reset while the X-MASS communicates with the Disk-On-Module. If all is good the reset is released and the CPC starts up.