got 'device unaccessible' in the middle of transferring pictures from iphone 7 to Windows 10 PC

Use Windows Photo App -- The process is given below

Import Photos and Videos to your PC

You can import photos to your PC by connecting your device to your computer and using Windows Photos app:

1. Make sure that you have the latest version of iTunes on your PC. Importing photos to your PC requires iTunes 12.5.1 or later.
2. Connect your iPhone, iPad, or iPod touch to your PC with a USB cable.
3. You might need to unlock your iOS device using your passcode.
4. You might also see a prompt on the iOS device asking you to Trust This Computer. Tap Trust or Allow to continue.

Then, follow the steps in these Microsoft knowledge base articles to learn how to import photos to Windows Photos app:

• Windows 10
• Windows 8
• Windows 7

When you import videos from your iOS device to your PC, some might be rotated incorrectly in Windows Photos app. You can add these videos to iTunes to play them in the correct orientation.

If you have iCloud Photos turned on, you need to download the original, full resolution versions of your photos to your iPhone before you import to your PC. Find out how.

Photos and videos that you sync from your computer to your iOS device using iTunes can't be imported back to your computer. 

Learn what to do if you can't import photos from your iPhone, iPad, or iPod touch to your computer.