Addressbooks are retained in the profile, which is normally on drive C. It is possible to locate the profile on another drive with the Profile Manager (or by twiddling with the profiles.ini file), but many (possibly you?) locate their accounts on other drives, but leave the profile on drive C. So, the addressbooks are wherever you place the profile.

It would help to tell me the name of the address file(s).  :-) It seems, despite what this forum says elsewhere, that *.MAB files are no longer used and *.SQLite DB is now the format. The only files named *.MAB I can find on my laptop & backups are actually *.MAB.BAK dated 10 years ago.

The profiles on the D: drive hold all the messages & folders - which is what I hoped for - but no *.SQLite files. In Thunderbird the address book is the same for all accounts/profiles: whether Business, Bike Club or Personal.

The only abook.SQLite file I can find is in C:\users\...\AppData\Roaming\Thunderbird\Profiles\h7blkrmm.default. It's not readable - the tag-names are OK but the data are encrypted - yet it fits the bill

My conclusion is that this is the answer. Thunderbird's address book sits in the default account on the C: drive (and cannot be relocated) regardless of where the account profiles are kept and is available to ALL accounts. But it doesn't get backed up with the profiles.

Q1 can anyone confirm this? Q2 can someone explain why Thunderbird chose to do it this way?

Deeply puzzled, Jem Moore

I will try to be more clear. The addressbook sits in the profile,wherever the profile sits. The profile normailly also contains the account message folders, but those can be relocated to other drives when desired, as can the profile itself. The name of addressbook is abook.sqlite, and there are variations that may appear, such as abookv3.sqlite. Together, they all represent the addressbook. Yes, it is used by all accounts. (I think the name change from abook.mab happened with the release of ver 78.)

David, thank you very much, that's much clearer. My mistake is to think profile is the same as account. The profile is per user (I can only talk about Windows for PCs) whereas one user may have a number of accounts. Is that correct? By default both profile & accounts are stored in C:\users\...\AppData\Roaming\Thunderbird\Profiles\... but the account data folder can be placed elsewhere when it is created. You mention that the profile can be located on other drives. How can I do that? Is that defined when Thunderbird is installed? I'd like to do that simply because it makes my back-up procedures easier. Thanks again for bearing with me

Here is info on moving the profile: https://kb.mozillazine.org/Moving_your_profile_folder_-_Thunderbird

basically, here is how I do it (but keep the URL for reference): - exit TB - copy profile to wherever you want it -click windows key an 'r' key - in that window, enter thunderbird.exe -P and press enter. <== that is an uppercase P - you are now in profile manager, click create profile - click next and then enter a desired name, such as MYPROFILE, - now, click the browse button to locate and select the place where you copied profile. - continue to finish and then restart Thunderbird - if all is okay, you can then delete the original profile if desired