The phone (LG G4) is connected via USB cable. The internal storage is available in file manager.

[![Internal storage](https://i.stack.imgur.com/e9WhC.png)](https://i.stack.imgur.com/e9WhC.png)

I am unable to add the storage to Virtualbox. I have tried Creating new disk and Choosing existing disk, but cannot find the phone storage.

[![Disk](https://i.stack.imgur.com/Y0GbY.png)](https://i.stack.imgur.com/Y0GbY.png)

Goal is to run SpinRite on the very slow internal storage. I have only this guide to go by, but I am unable to copy the procedure.

"Using Virtual Box on Linux, I plugged in the phone to the USB, had Virtual Box set it as a raw disk, spun up a VM with SpinRite and pointed it at the cell phone. SpinRite did it's thing, and about an hour later it was done, I unplugged the phone, rebooted it, and after that it was running faster and more stable than it had for over a year! If the phone can be seen as a mass storage drive over its USB port, SpinRite can fix it!"
[Source](https://www.grc.com/sr/testimonials.htm)