While listed as only for a 2.5 HDD, has anyone tested this on a Desktop 3.5 HDD?

This will not work on a 3.5 drive. The USB power supply is 5V. The 2.5 drives only use a 5V power rail, which is why this works on them. 3.5 drives have a 12V power rail, along with the 5V power rail. To run a 3.5 drive, you need an external 12V power supply. There are interfaces that come with it.

USB 3.0 to 2.5" SATA III Hard Drive Adapter Cable w/UASP ... Great! Is this thing USB 2.0 backward compatible in case I need it that way?

I've been able to use these with SSD's and USB 2.0. Usually laptop HDD's pull too much power to work on 2.0, and I've seen a couple older ones pull too much for 3.0. I work in a computer repair shop where we use these things like crazy, and it often depends on the drive you're using, and whether your USB port is able to push the power it's supposed to be rated for. Feels like a coin flip if you're working with a platter drive, but SSD's do fine.

2.5" drives don't require a power adaptor? All I need is this cable?

Yes. I had an SSD drive connected to the back of a server's USB 3 port for weeks without issue with no external power adapter.

Is this compatible with the Samsung Data Migration / Magician tools? They will not detect my drive with the other adapters I own.

I just ran into this issue with a Samsung 500 Gb 860 EVO SSD. The Samsung Magician Software did not see the SSD. There was a suggestion on one of Samsung's support sites (user community) that the bios settings for the HDD controller could be at fault. I checked mine and they were set to RAID (even though I don't have a RAID configuration). The recommended setting is AHCI. I changed it to AHCI (despite the warning that the computer might fail to boot) and rebooted. Rebooted without an issue, but Magician still didn't "recognize" the SSD as a Samsung "compatible" drive. I then downloaded Samsung Data Migration tool, which is really what you need for the cloning. IT WORKED. So as indicated elsewhere, there is some issue with certain combinations of Samsung software and the newer SSD's. I will try to remember to test it with an older USB 2.0 -> SATA adapter I've used in the past without issue. In the end, the cable worked as advertised.

Brand	StarTech.com
Product Dimensions	50.04 x 4.83 x 1.02 cm; 40.82 g
Batteries	1 AAAA batteries required.
Item model number	USB3S2SAT3CB
Manufacturer	Startech
Series	SATA/USB Data Transfer/Power Cable
Colour	Black
Standing screen display size	2.5 Inches
Processor Brand	Refer_Manual
Processor Type	Unknown
Hard Drive Interface	ATA
Wireless Type	Radio Frequency
Number of USB 3.0 Ports	3
Wattage	7.5 watts
Power Source	AC
Hardware Platform	Refer_Manual
Are Batteries Included	No
Lithium Battery Energy Content	2.6 Watt Hours
Lithium Battery Packaging	Batteries contained in equipment
Lithium Battery Weight	5 g
Number Of Lithium Ion Cells	4
Item Weight	40.8 g
Guaranteed software updates until	unknown

StarTech.com SATA to USB Cable - USB 3.0 to 2.5” SATA III Hard Drive Adapter - External Converter for SSD/HDD Data Transfer (USB3S2SAT3CB)

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