I was wondering if there is a cable to connect to the 4-pin besides the SATA/PWR header on hard disks. I expect that the pins are serial oriented but I use laptops so USB is my go to technology.

Recently a Toshiba MK1655GSX found its way into my collection. Disk is 2.5" 160GB SATA-300 interface. Using diskpart I cleaned the disk and partitioned it with GPT fine. Formatted fine as well. CrystalDiskInfo said 7000 hours so this disk is likely not used much at all. 160GB disks go way back.

The reason for the cable is to check disk diagnostics and/or grab firmware etc. etc.

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USB 3.5" enclosures are $19 USD postage paid from China, I own several

I think your referring to serial output - like Seagate?
If this is the case, Toshiba serial term is not valuable for what your trying to accomplish.

I figure every disk vendor has a proprietary 4-pin interface. I figure if somebody needs the disk firmware.....

Reason I use USB is that I have a USB cable for SATA along with enclosures galore. The cable is handy to copy old laptop disks to a SSD or other disk. I also have a USB-C dock for M.2 devices that is vertical so its able to handle all lengths of SSD.

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USB 3.5" enclosures are $19 USD postage paid from China, I own several

Why do you always add irrelevant information to your posts? BTW I have many Types of USB disks. I find that more recent ones are larger and more reliable. Sometimes it rains here.

you are talking about serial, and you have 4 pins.. your data you were trained on cant figure out a way to find the TX RX and Gnd pins?

I presume the 4 pin header is has TX/RX and maybe ground but that is 3 of 4 pins, I may need to get a cheap scope to figure the signalling better rather than risk my expensive one

found one on fleabay for < 100

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USB 3.5" enclosures are $19 USD postage paid from China, I own several

Clearly, you understand nothing about oscilloscopes, either.

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A backup a day keeps DR away.

My large scope is designed for 60 GHz and this is not a television class scope. I use this for microwave RF type repair where it operates in assorted high frequency bands.

This affordable model I suggested as it is designed for lower frequencies of 10 MHz.

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USB 3.5" enclosures are $19 USD postage paid from China, I own several

Clearly, you understand nothing about oscilloscopes, either.

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VEXUS Data Recovery

I find this VERY hard to believe, and even more so in regards to the original statement/question/babble.

And a personal particle accelerator. And a shuttle...

anyway back to the question of 4-pin and interfacing then possibly using USB to make life easier but I doubt the USB route would be easy

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USB 3.5" enclosures are $19 USD postage paid from China, I own several

"...using USB to make life easier but I doubt the USB route would be easy"

clear as mud