diff --git a/src/devices/machine/nscsi_bus.cpp b/src/devices/machine/nscsi_bus.cpp
index a16ff1f5b03..ef3c0dd9f9a 100644
--- a/src/devices/machine/nscsi_bus.cpp
+++ b/src/devices/machine/nscsi_bus.cpp
@@ -442,7 +442,7 @@ void nscsi_full_device::step(bool timeout)
 			break;
 		}
 
-		if(command_done()) {
+		if(scsi_command_done(scsi_cmdbuf[0], data_buffer_pos)) {
 			scsi_cmdsize = data_buffer_pos;
 			scsi_bus->ctrl_wait(scsi_refid, 0, S_ACK);
 			scsi_command();
@@ -509,18 +509,17 @@ void nscsi_full_device::target_send_buffer_byte()
 	target_send_byte(scsi_get_data(data_buffer_id, data_buffer_pos++));
 }
 
-bool nscsi_full_device::command_done()
+bool nscsi_full_device::scsi_command_done(uint8_t command, uint8_t length)
 {
-	if(!data_buffer_pos)
+	if(!length)
 		return false;
-	uint8_t h = scsi_cmdbuf[0];
-	switch(h >> 5) {
-	case 0: return data_buffer_pos == 6;
-	case 1: return data_buffer_pos == 10;
-	case 2: return data_buffer_pos == 10;
+	switch(command >> 5) {
+	case 0: return length == 6;
+	case 1: return length == 10;
+	case 2: return length == 10;
 	case 3: return true;
 	case 4: return true;
-	case 5: return data_buffer_pos == 12;
+	case 5: return length == 12;
 	case 6: return true;
 	case 7: return true;
 	}
diff --git a/src/devices/machine/nscsi_bus.h b/src/devices/machine/nscsi_bus.h
index f3404ceb526..244eda94fe8 100644
--- a/src/devices/machine/nscsi_bus.h
+++ b/src/devices/machine/nscsi_bus.h
@@ -308,6 +308,7 @@ protected:
 
 	virtual void scsi_message();
 	virtual void scsi_command();
+	virtual bool scsi_command_done(uint8_t command, uint8_t length);
 
 	void scsi_unknown_command();
 	void scsi_status_complete(uint8_t st);
@@ -448,7 +449,6 @@ private:
 	void target_recv_byte();
 	void target_send_byte(uint8_t val);
 	void target_send_buffer_byte();
-	bool command_done();
 };
 
 
diff --git a/src/devices/machine/nscsi_cd.cpp b/src/devices/machine/nscsi_cd.cpp
index 186f22b1704..06ecb71bb04 100644
--- a/src/devices/machine/nscsi_cd.cpp
+++ b/src/devices/machine/nscsi_cd.cpp
@@ -3,10 +3,11 @@
 #include "emu.h"
 #include "machine/nscsi_cd.h"
 
-#define VERBOSE 1
+#define VERBOSE 0
 #include "logmacro.h"
 
 DEFINE_DEVICE_TYPE(NSCSI_CDROM, nscsi_cdrom_device, "scsi_cdrom", "SCSI CD-ROM")
+DEFINE_DEVICE_TYPE(NSCSI_CDROM_SGI, nscsi_cdrom_sgi_device, "scsi_cdrom_sgi", "SCSI CD-ROM SGI")
 DEFINE_DEVICE_TYPE(NSCSI_RRD45, nscsi_dec_rrd45_device, "nrrd45", "RRD45 CD-ROM (New)")
 DEFINE_DEVICE_TYPE(NSCSI_XM3301, nscsi_toshiba_xm3301_device, "nxm3301", "XM-3301TA CD-ROM (New)")
 DEFINE_DEVICE_TYPE(NSCSI_XM5301SUN, nscsi_toshiba_xm5301_sun_device, "nxm5301sun", "XM-5301B Sun 4x CD-ROM (New)")
@@ -19,6 +20,11 @@ nscsi_cdrom_device::nscsi_cdrom_device(const machine_config &mconfig, const char
 {
 }
 
+nscsi_cdrom_sgi_device::nscsi_cdrom_sgi_device(const machine_config &mconfig, const char *tag, device_t *owner, uint32_t clock) :
+	nscsi_cdrom_device(mconfig, NSCSI_CDROM_SGI, tag, owner, "Sony", "CDU-76S", "1.0", 0x00, 0x05)
+{
+}
+
 nscsi_dec_rrd45_device::nscsi_dec_rrd45_device(const machine_config &mconfig, const char *tag, device_t *owner, uint32_t clock) :
 	nscsi_cdrom_device(mconfig, NSCSI_RRD45, tag, owner, "DEC     ", "RRD45   (C) DEC ", "0436", 0x98, 0x02)
 {
@@ -559,3 +565,46 @@ void nscsi_cdrom_device::scsi_command()
 		break;
 	}
 }
+
+enum sgi_scsi_command_e : uint8_t {
+	/*
+	 * The SGI supplied CD-ROM drives (and possibly those from some other vendors)
+	 * identify themselves as hard disk drives at poweron, and after SCSI bus resets,
+	 * until issued a vendor specific command (0xc9).  This is done because older
+	 * systems would otherwise be unable to boot and load miniroots from CD, due to
+	 * their design (they attempted to protect the user from booting from
+	 * "ridiculous" devices, long before CD-ROM drives existed).  The SGI drives are
+	 * sent a command to "revert" to CD-ROM inquiry information during boot if on
+	 * a SCSI bus handled by the PROM, but not all possible buses are handled by all
+	 * PROMs; additionally, a SCSI bus reset causes the CD-ROM drives to revert to
+	 * the poweron default, and this could happen before the hardware inventory code
+	 * in the kernel runs, if there are SCSI problems.
+	 */
+	SGI_HD2CDROM = 0xc9,
+};
+
+void nscsi_cdrom_sgi_device::scsi_command()
+{
+	switch (scsi_cmdbuf[0]) {
+	case SGI_HD2CDROM:
+		LOG("command SGI_HD2CDROM");
+		// No need to do anything (yet). Just acknowledge the command.
+		scsi_status_complete(SS_GOOD);
+		break;
+
+	default:
+		nscsi_cdrom_device::scsi_command();
+		break;
+	}
+}
+
+bool nscsi_cdrom_sgi_device::scsi_command_done(uint8_t command, uint8_t length)
+{
+	switch (command) {
+	case SGI_HD2CDROM:
+		return length == 10;
+
+	default:
+		return nscsi_full_device::scsi_command_done(command, length);
+	}
+}
\ No newline at end of file
diff --git a/src/devices/machine/nscsi_cd.h b/src/devices/machine/nscsi_cd.h
index 3a5f67b7170..48097cea42f 100644
--- a/src/devices/machine/nscsi_cd.h
+++ b/src/devices/machine/nscsi_cd.h
@@ -57,6 +57,16 @@ private:
 	static int to_msf(int frame);
 };
 
+class nscsi_cdrom_sgi_device : public nscsi_cdrom_device
+{
+public:
+	nscsi_cdrom_sgi_device(const machine_config &mconfig, const char *tag, device_t *owner, uint32_t clock = 0);
+
+protected:
+	virtual void scsi_command() override;
+	virtual bool scsi_command_done(uint8_t command, uint8_t length) override;
+};
+
 class nscsi_dec_rrd45_device : public nscsi_cdrom_device
 {
 public:
@@ -94,6 +104,7 @@ public:
 };
 
 DECLARE_DEVICE_TYPE(NSCSI_CDROM, nscsi_cdrom_device)
+DECLARE_DEVICE_TYPE(NSCSI_CDROM_SGI, nscsi_cdrom_sgi_device)
 DECLARE_DEVICE_TYPE(NSCSI_RRD45, nscsi_dec_rrd45_device)
 DECLARE_DEVICE_TYPE(NSCSI_XM3301, nscsi_toshiba_xm3301_device)
 DECLARE_DEVICE_TYPE(NSCSI_XM5301SUN, nscsi_toshiba_xm5301_sun_device)
diff --git a/src/mame/drivers/indy_indigo2.cpp b/src/mame/drivers/indy_indigo2.cpp
index bd75c7c3a5b..882b167bad7 100644
--- a/src/mame/drivers/indy_indigo2.cpp
+++ b/src/mame/drivers/indy_indigo2.cpp
@@ -200,7 +200,7 @@ void ip22_state::wd33c93(device_t *device)
 
 void ip22_state::scsi_devices(device_slot_interface &device)
 {
-	device.option_add("cdrom", NSCSI_CDROM);
+	device.option_add("cdrom", NSCSI_CDROM_SGI);
 	device.option_add("harddisk", NSCSI_HARDDISK);
 	//device.set_option_machine_config("cdrom", cdrom_config);
 }
diff --git a/src/mame/machine/hpc1.cpp b/src/mame/machine/hpc1.cpp
index c7c5bb3d619..75ec6f368f5 100644
--- a/src/mame/machine/hpc1.cpp
+++ b/src/mame/machine/hpc1.cpp
@@ -123,7 +123,7 @@ void hpc1_device::indigo_mice(device_slot_interface &device)
 
 void hpc1_device::scsi_devices(device_slot_interface &device)
 {
-	device.option_add("cdrom", NSCSI_CDROM);
+	device.option_add("cdrom", NSCSI_CDROM_SGI);
 	device.option_add("harddisk", NSCSI_HARDDISK);
 }