[HTC One S] :
I'd like to thank both of you for this moment of glory! :cowboy:

I'm running

Windows 10 Pro x64 on a Notebook

with Intel Hasswell-CPU, that has

only USB 3.0-Ports and tried to connect it to my

HTC Legend on B 0.8.4

in fastboot-mode,​

absolutely in vain for quite a long time. Now these days are over.

I've finally got it, ... after several attempts on different days for several hours.

And of course I'd like to share my procedure with you, even though I can't tell which was the crucial part of it.

attempt = the simple way: automatical tools/software

I should keep focused on the things that did work in the end, right? Right.

But let's really start primordial:

My phone was already rooted, the bootloader was unlocked and CWM Recovery was installed, so I could enter the fastboot-mode on the phone straight from the beginning of the process. I could flash custom Roms too, via CWM Recovery. I tried to get fastboot running on the PC. "Minimal ADB and Fastboot" did the job at it's best. It's way smaller than a whole "Android SDK" package would be.

Lots of youth naiveté led me the way to think, actual phone drivers won't be needed, because modern smartphones are surely overall "plug & play"! Surely ... :p

So I put phone and computer together and what happened? Of course, nothing.

At least ADB drivers / Google USB drivers are necessary, to address any Android's Debug Bridge.

But even therefor exists an automatized way: "ADB Driver Installer". I have no clue if it worked. I do know that there was still no connection between computer and phone in fastboot-mode, although "ADB Driver Installer" displayed "driver already installed", in mode of ROM-operation. Mode of ROM-operation? I mean like beeing on the homescreen of your (Custom) ROM, phone connected to the PC.

"HTC Sync 3.3.63" (the PC software) detected the phone as well. I installed it alongside "HTC Driver 3.0.0.007", "HTC Driver 4.0.1.001", "HTC Mobile Driver 4.17.0.001", "HTC Sync Manager 3.1.77.0". You definitely don't need more than one driver package. I only named all driver installer to illustrate, how desperate I was searching for a solution.

Important note: You probably already knew, that USB-Debugging has to be switched "on". You might not know, until now, that your phone must neither be connected as "charge only" nor as "mass storage". Unfortunately different Android OS' offer different (and sometimes many) connection types. My current Custom Rom allows four types of connection: "charge only", "hard disk drive" (= "mass storage"?), "USB-connection" (= ?MTP? or ?PTP?), "HTC Sync". The last one is the one to take. I tried that approach with CyanogenMod ROMs (CM 7.2.0, CM 9.1.0 BETA8, CM 10.0.0 BETA1) before, but, for some reason or another, I wasn't able to set the right type of connection.

Second important note: Assumingly, it's not necessary - for to get a working fastboot-connection - to get a working connection in mode of ROM-operation before. Well, I'm not sure about it, so I achieved this kind of connection too and include the description here, just for the record. Isn't it nice to have anyway? :cool:

attempt = installing - manually - all available drivers

Where are we now? Finally the phone gets successfully detected by "HTC Sync 3.3.63", whenever we connect it to the PC in mode of ROM-operation, USB-Debugging switched "on", type of connection set to "HTC Sync" (or the comparable type of your phone/ROM).

What about fastboot? It still doesn't detect the phone whenever we start a command prompt in the root directory of "Minimal ADB and Fastboot", phone connected in fastboot mode, typing

fastboot devices

What to do next? I suppose the HTC drivers are working properly now, concluding ADB drivers / Google USB drivers maybe not. Possibly, "ADB Driver Installer" didn't do the trick. Let's continue doing it manually.

"Universal ADB Drivers" can be taken, to place ADB / Google USB driver-files into a folder of your choice. Various forum posts advised me to disable Windows' "driver signature enforcement" temporarily, before installing these driver-files via "Device Manager".

Another important note: I think it is an essential step for the whole procedure of connecting any (old) HTC device to a Windows (8, 8.1 or 10) PC via fastboot, to turn off the "driver signature enforcement" temporarily, each and every time, before you try to install ADB / Google USB driver-files manually via the "Device Manager". Gourav posted how it's to be done, at http://myandroidteacher.blogspot.de. We will use more of his instructions shortly after, but let's start with his explanations of how to turn the "driver signature enforcement" temporarily "off".

Now install ADB / Google USB driver-files manually via the "Device Manager", following this video instruction. I know it's quality is not the best, but these are the steps my humble self did and that's all I can serve you here.

I don't know if the former steps made any sense at all, but the following ones do for sure. At least in case you have an Intel CPU and USB 3.0 ports. If you have USB 2.0 ports (as well), you should try your fastboot connection with them first and if you don't have an Intel CPU, the whole problem (described as follows) should not occur, which basically means, that the next steps won't solve your connecting problems. They solved mine.

Gourav writes, the issues, causing ADB drivers / Google USB drivers not to work, can have to do with Microsoft USB 3.0 drivers on Intel USB Host Controller(s). He wrote a workaround to fix this and I followed it carefully, which finally led me to a working fastboot connection between my PC and my phone. So I'd recommend you to follow it too.

One last important note: The two required files ("iusb3xhc.inf" and "iusb3hub.inf"), Gourav mentions, are not accessible through his links at the moment, so I attach them to this post.