I am using SCSIPeripheralsDriverKit/IOUserSCSIPeripheralDeviceType00 to develop a Dext driver, which has only two classes: MyUserSCSIPeripheralDeviceType00 and MyUserClient, MyUserClient is responsible for receiving APP commands and passing them to MyUserSCSIPeripheralDeviceType00->UserSendCDB(), After the device is connected to the computer, the driver can match the device and start, calling MyUserSCSIPeripheralDeviceType00-->init(), MyUserSCSIPeripheralDeviceType00-->Start(), Any command sent by the APP to MyUserSCSIPeripheralDeviceType00 will return a failure: kIOReturnUnsupported (0xe00002c7), including UserSendCDB(), UserReportMediumBlockSize(), <!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd"> <plist version="1.0"> <dict> <key>IOKitPersonalities</key> <dict> <key>MySCSIDriver</key> <dict> <key>CFBundleIdentifier</key> <string>$(PRODUCT_BUNDLE_IDENTIFIER)</string> <key>CFBundleIdentifierKernel</key> <string>com.apple.kpi.iokit</string> <key>IOClass</key> <string>IOUserService</string> <key>IOMatchCategory</key> <string>$(PRODUCT_BUNDLE_IDENTIFIER)</string> <key>IOProviderClass</key> <string>IOSCSIPeripheralDeviceNub</string> <key>IOResourceMatch</key> <string>IOKit</string> <key>IOUserClass</key> <string>MyUserSCSIPeripheralDeviceType00</string> <key>IOUserServerName</key> <string>$(PRODUCT_BUNDLE_IDENTIFIER)</string> <key>UserClientProperties</key> <dict> <key>IOClass</key> <string>IOUserUserClient</string> <key>IOUserClass</key> <string>MyUserClient</string> </dict> <key>bConfigurationValue</key> <integer>1</integer> <key>bInterfaceNumber</key> <integer>0</integer> <key>bcdDevice</key> <integer>1</integer> <key>idProduct</key> <string>*</string> <key>idVendor</key> <integer>12345</integer> <key>Peripheral Device Type</key> <integer>0</integer> <key>IOKitDebug</key> <integer>65535</integer> </dict> </dict> </dict> </plist> This is the method that receives client calls kern_return_t MyUserSCSIPeripheralDeviceType00::HandleExternalStruct(IOUserClientMethodArguments* arguments) { kern_return_t ret = kIOReturnSuccess; SCSIType00OutParameters command; SCSIType00InParameters response; SCSI_Sense_Data sense; UInt64 fSenseAddr = (intptr_t)&sense; bzero((void *)&command, sizeof(SCSIType00OutParameters)); bzero((void *)&response, sizeof(SCSIType00InParameters)); bool test = TEST_UNIT_READY(&command, &response, fSenseAddr); // test is true ret = this->UserSendCDB(command, &response); // ret is kIOReturnUnsupported （0xe00002c7） UInt64 blockSize; kern_return_t sizeRet = this->UserReportMediumBlockSize(&blockSize); // sizeRet is kIOReturnUnsupported （0xe00002c7） }; // Not called. As per the documentation, this should be called during enumeration. kern_return_t IMPL(I4UserSCSIPeripheralDeviceType00, UserDetermineDeviceCharacteristics) { // *result = true; return kIOReturnSuccess; }