Mounting MB Air as an External Drive on Another Directly-Connected Laptop

So as to mount one MB Air as an external drive on another MB Air, I connected the two directly and started the target with Cmd-T.  It displayed the lightning rod pictogram on its display and came up as a volume on the Host in Finder windows’ sidebars.

(I had earlier done an Erase in Disk Utility on the target laptop and decided to do a diskutil secureerase on it.) 

Now to avoid typing and also avoid making a mistake, I drag disks, folders, etc. from Finder windows’ title bars into Terminals (to run some command).  In this instance I carelessly dragged  it from a Finder window’s sidebar with the inevitable result of it vanishing from the sidebar – and apparently everywhere else.

Here’s what I tried to get it back:–

1.  Shutting down and restarting the target with Cmd-T.

2.  Trying to find, mount, or open it in Disk Utility of the host.

3.  Doing ls /Volumes/ in Terminal of the host

4.  Seeing if it’s on the desktop of the host

5.  Checking/ticking options in Finder Settings of the host

6.  Finder menu’s ‘Go’ of the host

None of it worked.  I could try to mount it in Terminal but I don’t know what to mount.(!)  (After trying all this I wonder if it even appeared as an external drive the first time and whether I did not drag something else off the sidebar.  That said, it does start up in target mode, displaying the lightning bolt.) 

How can I get that MB Air to mount as an external drive again (assuming that it did the first time around)?

[Re-Titled by Moderator]