Ah, this is one of those "I wouldn't start from here" situations. Had you asked at the time, you would have been guided towards setting up Thunderbird to use your old profile. Right now, I guess you have a big older profile and a new smaller one. I would set Thunderbird to use the old profile, then use the ImportExportTools addon to import the required folders into it from the new smaller profile. I take it that you are or were using POP, or were making regular use of Local Folders, because if you were using IMAP this question doesn't really arise; you would already have all your old messages. The ImportExportTools addon will allow you to import into Local Folders, and possibly also into accounts using POP. You may have to settle for importing your old messages into a new temporary folder in Local Folders and then move them manually to where to really want them to be. That's my reason for importing the new into the old: there is less material to move around when doing the tidying up and reconciliation. One other thing to decide; are you happy for the working profile to be on D, or would you want it on the new computer's own hard drive? Decide this and do it before combining the two profiles. If you wish to leave it on D then we adjust the settings, by means of editing the profiles.ini file, or using the Profile Manager, or by use of the Profile Switcher add on. If you want it on C: then you'll need to copy from D: beforehand.