If you have a volume, lets say D:, and then remove the drive letter, the volume still shows up if you use 'Get-Volume'.
The volume of course shows up with no drive letter but you can see it's UniqueID, eg:
You can then copy and paste this UniqueID into a folder explorer and browse the volume with no problem.
Is there a way to "cd", or something similar, into this disk in PowerShell?
The question in short:
I need to access the files on a disk, via PowerShell, with no drive letter assigned to it.
You could try mapping a PSDrive (which would not be a "drive letter" assignment), using that "path" as the root. But PS can't "change" to anything not represented by a PSDrive (which is not the same as a system-level drive letter on a volume).