It's interesting that you weren't able/willing to track down the source of the problem. It's also interesting how you mention "single drag and drop". In my experience, you can't drag/drop _folders_ in Thunderbird at all. All the migrations I've done this way required
I migrated to my new server by drag/drop from Thunderbird. Using IMAP. Using Thunderbird 2.0.0.24. No command line needed to convert from server to server, all done via the GUI of the client.
Original server was mbox, destination is dovecot with maildirs.
The ONLY problem I had was with folders which contained a "dot" in the name. An old folder called "2010-june.providers" was understood as a folder called "providers" inside a folder called "2010-june" due to the name convention used in the dovecot server. The copy failed there and the destination dir was not behaving well.
Just deleted the erroneous destination folder, I renamed to something like "2010-june_providers" and retried from there on.
I have migrated a 3 GB account (mine) in this way.
For security I did not do "all at a time". I just moved 10 folders, upon finish 10 more folders and so on.
TB handles well if the destination folder exists. Example:
Supose I have:
old/2001 to old/2009
Spoose in the DST you already have old/1996 to old/2000
By dragging "old" onto "old" it will copy from 2001 to 2009 inside "old" at the same level than 1996 to 2000.
NEVERTHELESS IT IS TRUE that sometimes TB refuses to do the drag-n-drop. At this time, just close TB and reopen it and Drag-n-drop worked again.
Hope to help. Xavi.
-- Xavier Montero - 93 589 71 91 - 630 59 01 62 - xmontero@dsitelecom.com