Question External Drive - what does the PC see?

wildfox · Nov 6, 2024

I have a variety of external drives which use SATA and PCIe interfaces, and include conventional hard drives, thumb drives, and NVMe SSD's.

When I connect any of these drives to my PC via the USB or Thunderbolt ports, does my PC have "awareness" or care what type of external drive it is? Or does it simply appear as a data stream from the port and my PC doesn't care about the technical architecture of the drive?

fzabkar · Nov 6, 2024

Your PC sees a USB mass storage device and communicates with it using SCSI commands.

lakedude · Nov 6, 2024

fzabkar said:
Your PC sees a USB mass storage device and communicates with it using SCSI commands.

SCSI, really?

lakedude · Nov 6, 2024

wildfox said:
I have a variety of external drives which use SATA and PCIe interfaces, and include conventional hard drives, thumb drives, and NVMe SSD's.

When I connect any of these drives to my PC via the USB or Thunderbolt ports, does my PC have "awareness" or care what type of external drive it is? Or does it simply appear as a data stream from the port and my PC doesn't care about the technical architecture of the drive?

I don't know, but it has been a long time since the computer needed to know the proper number of heads/cylinders/sectors etc. a hard drive has. Quite a while back a drive's controller took over the job of managing the physical details and then it communicates with the computer using a standardized format that has nothing to do with the physical layout of the drive

wildfox · Nov 7, 2024

fzabkar said:
Your PC sees a USB mass storage device and communicates with it using SCSI commands.

Thanks, you reminded me that I can look at the Device Manager to see what info it provides on active USB connections.

For USB2 and USB3.x, the connections show as you indicated. The PC has no awareness of the external drive's interface as the data stream gets wrapped in SCSI for transfer via USB.

For my Thunderbolt 3 port, it's different. I can see an entry under System Devices, not the USB section. Apparently, the PCIe interface of an SSD in a Thunderbolt 3 drive enclosure is directly supported by the PC for faster speeds.

tcsenter · Nov 7, 2024

wildfox said:
When I connect any of these drives to my PC via the USB or Thunderbolt ports, does my PC have "awareness" or care what type of external drive it is? Or does it simply appear as a data stream from the port and my PC doesn't care about the technical architecture of the drive?

The system does not know or care. It's an abstraction layer. The controller chip in the external enclosure exposes the storage device to the system. The system just sees mass storage device but there are features or capability the OS can certainly be 'aware' of.

kschendel · Nov 7, 2024

lakedude said:
SCSI, really?

Yep, assuming the USB-to-SATA chip in the enclosure supports UASP. I haven't seen a non-UASP enclosure for a while (outside of the $5 junk).

Jimminy · Nov 8, 2024

wildfox said:
Thanks, you reminded me that I can look at the Device Manager to see what info it provides on active USB connections.

For USB2 and USB3.x, the connections show as you indicated. The PC has no awareness of the external drive's interface as the data stream gets wrapped in SCSI for transfer via USB.

For my Thunderbolt 3 port, it's different. I can see an entry under System Devices, not the USB section. Apparently, the PCIe interface of an SSD in a Thunderbolt 3 drive enclosure is directly supported by the PC for faster speeds.

You might like the usb tree view utility. It give a lot of info on any type of usb device connected, (although windows don't care about the details). It can come in handy when troubleshooting USB devices. It's free too. Here's the web page:

USB Device Tree Viewer

igor_kavinski · Saturday at 11:36 AM

fzabkar said:
Your PC sees a USB mass storage device and communicates with it using SCSI commands.

Correct. https://www.startech.com/en-us/blog/all-you-need-to-know-about-uasp

Question External Drive - what does the PC see?

wildfox

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fzabkar

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lakedude

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lakedude

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tcsenter

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kschendel

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Jimminy

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igor_kavinski

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