The ADTPro server can host a virtual hard drive to a connected Apple II or /// machine running a driver from the distribution disk, VDRIVE-1.2.7.DSK. This is a different function from the usual disk transfer mechanics of the ADTPro client. With the VDrive driver running on an Apple, you can access an image named "Virtual.po" in the "Disks" folder of the ADTPro server. This Virtual.po disk image can be of whatever size and contents you like; and the connected client will be able to access the contents of that image via slot 2, drive 1 on an Apple II, and as the .VSDRIVE device on an Apple ///.
The ADTPro distribution comes with a 800k Virtual.po file pre-built for you. There's not much on it, as it's intended to give you a little extra storage space and to get used to how the virtual drive system works. You can use that one as-is, or you can substitute your own instead. If you would like to make your own blank one, CiderPress or AppleCommander can do that for you. For example, from the command line where you normally run ADTPro from, you could enter one of these commands, based on your platform:
ac.bat -pro800 disks\Virtual.po VOLNAME sh ac.sh -pro800 disks/Virtual.po VOLNAME
The VDRIVE-1.2.7.DSK distribution disk is bootable on both the Apple II and Apple /// computers. The file contents of the VDRIVE-1.2.7.DSK disk specific to the Apple II are as follows:
The file contents of the VDRIVE-1.2.7.DSK distribution disk specific to the Apple /// are as follows:
Once you've established your serial connection between your Apple II and the ADTPro host server, boot or otherwise access the VDRIVE-1.2.7.DSK image on the Apple II side. It will start with some brief instructions:

The serial VSDRIVE driver will automatically search for compatible hardware and use it. The slot that the serial adapter is in is independent of the slot that will be made available to the system as the virtual drive (S2D1). It is perfectly fine to have the serial adapter in physical slot 2 while the driver makes S2D1 available to the system.
There are two versions of the serial driver: VSDRIVE.LOW and VSDRIVE. The VSDRIVE.LOW version will install itself in the main memory of the Apple II, reducing the amount of memory available to BASIC and other programs. The ProDOS Utilities Disk will not work with this version of the driver. The VSDRIVE version will instead overwrite the Disk II driver, making Disk II floppies inaccessible to ProDOS. The ProDOS Utilities Disk is compatible with this version, and it will stay resident even when the ProDOS QUIT code is active.
On a machine with a typical Super Serial card adapter in slot 2 (or IIc/IIc+ or compatibles) this message will be displayed as it starts:

During normal operations with the virtual disk, activity will cause status messages to appear in the server's message areas:

Once you've established your Ethernet connection between your Apple II and the ADTPro host server, boot or otherwise access the VDRIVE-1.2.7.DSK image on the Apple II side. It will start with some brief instructions:

Before starting VEDRIVE for the first time, you should run VEDRIVE.CONFIG to set the IP address of the ADTPro server. This will re-write the VEDRIVE driver with IP address coded within it back to the disk. Then, starting the VEDRIVE driver will automatically search for compatible hardware and use it. You only need to set the IP address with VDRIVE.CONFIG once, or whenever your ADTPro server's IP address changes.
The slot that the Ethernet adapter is in is independent of the slot that will be made available to the system as the virtual drive (S2D1). It is perfectly fine to have the adapter in physical slot 2 while the driver makes S2D1 available to the system.
Here is a typical session configuring and starting the driver on a machine with an Uthernet adapter in slot 3:

When starting the VEDRIVE driver, you should notice a message "Ping request received from client" on the server if a connection has successfully been made. You can re-run the VEDRIVE command again if the ping message doesn't appear. If it continues to be a problem, re-check the IP address in the title bar of the ADTPro server and ensure you use that with VEDRIVE.CONFIG.

During normal operations with the virtual disk, activity will cause status messages to appear in the server's message areas:

Using the virtual serial drive on SOS requires you to add the VSDRIVE.A3DRVR device driver to your boot disk. The VDRIVE-1.2.7.DSK disk is bootable on the Apple ///, runs Business BASIC, and contains the required driver pre-installed for you. Once installed on any of your boot disks, the device ".VSDRIVE" will be available to the system, and will reflect the Virtual.po image served by the ADTPro server.

It is important to connect your null modem to the Apple /// and start the ADTPro server (and click the Serial button) before booting and using the serial driver. If you don't, changing the connectedness of the cabling while running causes SOS to crash with a $02 error.