調べたら下記のサイトが。
https://forums.virtualbox.org/viewtopic.php?f=6&t=48527
Cannot attach medium ‘C:\Program Files\Oracle\VirtualBox\VBoxGuestAdditions.iso’ {d6c41d7d-3727-4845-9aad-d89f671f7dc7}: medium is already associated with the current state of machine uuid {a2844f20-fc58-4d8b-8d01-d25a0358c487}!.
Result Code:
VBOX_E_OBJECT_IN_USE (0x80BB000C)
Component:
Medium
Interface:
IMedium {53f9cc0c-e0fd-40a5-a404-a7a5272082cd}Steps followed to resolve the issue.
1) Stop Oracle VM VirtualBox Manager
2) Open REAL1.vbox file in notepad
3) Search for VBoxGuestAdditions.iso in the file
4) Remove the line which corresponds which has the entry VBoxGuestAdditions.iso. (Remove the complete line)
eg:-
<Image uuid=”{0d0c7dc1-eee4-4f78-942c-7b62c4da37af}” location=”C:/Program Files/Oracle/VirtualBox/VBoxGuestAdditions.iso”/>
5) Start the Oracle VM VirtualBox Manager the VM machine should be available for startup.
無事解決。(^-^