Recently I had to convert a linked mailbox to a regular mailbox . I disabled the mailbox and to my horror I could not find the mailbox under disconnected mailboxes. First I decided to remain calm (that means PRAY the mailbox was not gone for ever)
I then turned to ‘Dr Google’ and after trial and error I found the key to sanity  . Below I now list the technical steps and relevant links. The following works with exchange 2007 /2010

According to technet I need to do a mailbox cleanup (this process is done on a schedule automatically after normal working hours)

Use the Clean-MailboxDatabase cmdlet to scan Active Directory for disconnected mailboxes that aren’t yet marked as disconnected in the Microsoft Exchange store and update the status of those mailboxes in the Exchange store. This cmdlet isn’t able to update the Exchange store unless the Microsoft Exchange Information Store service is running and the database is mounted.
syntax
clean-MailboxDatabase -Identity <DatabaseIdParameter> [-Confirm [<SwitchParameter>]] [-DomainController <Fqdn>] [-WhatIf [<SwitchParameter>]]

https://technet.microsoft.com/en-us/library/bb124076.aspx

This command syntax was a bit confusing so for clarity , below is the command I ended up typing (twice after my first refresh didn’t show anything)

Clean-MailboxDatabase -Identity mailboxservername/Databasename

The cursor is returned within a few seconds and I could now view the disconnected mailbox and proceeded with connecting the mailbox to a newly created windows account.