Just got bit in the ass when I ran a backup and clean out of my virtualbox directories,
scrupulously not deleting and/or altering in any way machines currently in use. After performing this virtuous act, I test booted my main machine. It didn't. Now approaching blind panic, I tried to figure out what I may have mistakenly deleted/moved. Nope, all present and accounted for. wtf? Finally I reimported the attached vdi's from the exact same location into virtualbox manager, and lo and behold, it booted.
EDITED: 2 May 15:52 by DSMITHHFX