The phone (LG G4) is connected via USB cable. The internal storage is available in file manager.
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.
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
You have to active Android MTP on your phone first. You active by turn on Developer Option in Setting function on your phone, and turn on USB debugging, after that select USB tethering on Default USB configuration.
SpinRite is not meant for flash or SSDs, in fact running it in write mode is bad. Adding in the fact that you are doing this on a phone over USB, this does not give you direct access to the disk. You are typically using MTP to communicate. Then add in that you are trying to do this over virtualbox, it makes me wonder whether the person claiming that they did this really did what they think they did.
At best, SpinRite will simply read all the files it can access over the USB, which is no different than just copying all of the files using the file manager. You will probably get better results doing a factory reset on the phone.
It would be best to Use the Devices tab from Virtual box, under the USB option select the Devices (Phone) you wish to connect to from the virtual machine, that would disconnect it from the host and pass control over to the virtual machine.