It is possible to unsling again without reflashing the slug.
I have done it on an unslung5.5 version.
You have to remove 2 files from the flash memory however.
Here is the relevant info from the Wiki.
telnet in and remove the “/.unslung” and “/.sd??root” files on the internal flash (/.sd?? being sda1 etc depending on the usb port your drive was connected to) reboot once again
telnet in again, attach the external flash drive, and “unsling” it again.
Failing to remove the files results in the SLUG appearing to unsling, but it will never boot, guess how I found out.
Not quite sure of this process on 6.8 beta though.
I know what you mean about the wiki not being the friendliest place to ask questions. 🙁
Hope this helps
You might consider as a backup in the future, the possibility off adding a second harddrive and doing a disk to disk backup (fix it first, see the wiki) between the 2 drives. You can then just switch the drives over in the event of a failure.