You aren't going to find it on your drive, but in a place in Photoshop that uses the pattern. Like if you go to Edit>Fill and toggle from Contents to say Foreground or Background Color to Pattern - the Pattern you made will be there. You can then create a file by creating a layer and filling it with the pattern fill and saving a jpg or whatever you need.
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You can see all your currently loaded patterns via Edit > Presets > Preset Manager [Preset Type = Patterns]. They are stored in the file C:\Users\[username]\AppData\Roaming\Adobe\Adobe Photoshop CC 2017\Adobe Photoshop CC 2017 Settings\Patterns.psp
While in the Preset Manager, if you select one or more patterns, you can save them as a set as a .PAT file, which is stored in the default location C:\Users\[username]\AppData\Roaming\Adobe\Adobe Photoshop CC 2017\Presets\Patterns, although you can store them in any location of your choosing.
Thank you that was great info.
Glad we helped.