Sorry if you already know some of the following information. I cannot tell from your message how much experience you have with Thunderbird.

The Thunderbird application and data are separate. Thunderbird on your Mac can access data on an external drive, so there is no need to install Thunderbird on an external drive. Just install it in Applications on your Mac.

Data is stored in profiles. You can create a profile on your external drive, import the mbox files by copying or moving them into your profile in the file system (not in Thunderbird, but you could do that too), and tell the Thunderbird app to use that profile.

how do I make sure TB knows that the data from the mbox(es) I import should "live" on the external drive as well?

I'm not sure that I understand. Are you expecting to end up with two instances of the data? After you import into Apple Mail, you will want only one instance of the data on the external drive. It will be in your Thunderbird profile and that will be the only place where Thunderbird accesses it.

Thunderbird never needs to know about any external e-mail accounts, including gmail.

Is this the approach that you want to take? You may need more information for each step of this process. Please ask again if you do, OK?

From my understanding you want to backup all your emails to your usb drive and keep only a limited number of emails on your local drive of your computer and your email provider is gmail.

If this is the case then you would need 2 profiles, one you already have on your local so you need to create a second profile (let's call it USB profile) and point that profile to your USB drive. The USB profile needs to be configured as POP3 mail (rather than IMAP) and "leave email on server" (do not delete). Since you did not delete any email yet, all you have to do is start another instance of Thunderbird with USB profile and download all your current emails. You need to edit your profiles.ini to tell thunderbirds that it is on USB drive. Unfortunately I don't work with Macs so I don't know the location of profiles.ini file.

Here is a sample line in profiles.ini on windows ( Path variable tells thunderbird the physical location of the profile, you need to substitute it with you Mac USB drive path ).

[Profile1] Name=USB-Drive IsRelative=0 Path=V:\appdata\Thunderbird\Profiles\default

Before deleting any email using the main profile, make sure that they are already downloaded on USB profile.

You can run 2 instances of Thunderbird at the same times.

You can run some tests before doing actual cleanup. Example : send to yourself a test email, make sure that it's been downloaded to USB profile. Then delete from your main profile. Go back to your USB profile to see if it's still there.

If this is the case then you would need 2 profiles, one you already have on your local so you need to create a second profile (let's call it USB profile) and point that profile to your USB drive. The USB profile needs to be configured as POP3 mail (rather than IMAP) and "leave email on server" (do not delete). Since you did not delete any email yet, all you have to do is start another instance of Thunderbird with USB profile and download all your current emails. Before deleting any email using profile 1, make sure that they are already downloaded on USB profile. You can run 2 instances of Thunderbird at the same times. You can run some tests before doing actual cleanup. Example : send to yourself a test email, make sure that it's been downloaded to USB profile. Then delete from your main profile. Go back to your USB profile to see if it's still there.

Hi Michael T. It is not clear that the OP has Thunderbird installed on the local drive already. On the local drive, the OP wants mail in Apple Mail, not Thunderbird. Downloading 91 GB of messages is not practical. Google Takeout is a better approach. I think that one profile and one instance will do the job.

Or have I misunderstood something?

Michel T said

If this is the case then you would need 2 profiles, one you already have on your local so you need to create a second profile (let's call it USB profile) and point that profile to your USB drive. The USB profile needs to be configured as POP3 mail (rather than IMAP) and "leave email on server" (do not delete). Since you did not delete any email yet, all you have to do is start another instance of Thunderbird with USB profile and download all your current emails. Before deleting any email using profile 1, make sure that they are already downloaded on USB profile. You can run 2 instances of Thunderbird at the same times. You can run some tests before doing actual cleanup. Example : send to yourself a test email, make sure that it's been downloaded to USB profile. Then delete from your main profile. Go back to your USB profile to see if it's still there.

Hi Michael T. It is not clear that the OP has Thunderbird installed on the local drive already. On the local drive, the OP wants mail in Apple Mail, not Thunderbird. Downloading 91 GB of messages is not practical. Google Takeout is a better approach. I think that one profile and one instance will do the job.

Or have I misunderstood something?

You're right, I misunderstood. I re-read and this is what I understood :

The OP wants to use Thunderbird on Mac with a profile residing on USB drive instead of local drive due to lack of space on the computer and have that profile as backup / archive of all emails "eternally" sort of speak . So yes the op would need only 1 profile. And you're right 91 Gb is a lot.

If I had a dollar for every time I misunderstood ...

If I had a dollar for every time I misunderstood ...

and this is how I became billionaire ...

Thanks so much for your responses!

@Rick, • I've never used Thunderbird before 😬 • I did find this support article – Profile Manager - Create and remove Thunderbird profiles – while researching solutions, which together with your response, helps give me clarity, thank you!

how do I make sure TB knows that the data from the mbox(es) I import should "live" on the external drive as well?

I'm not sure that I understand. Are you expecting to end up with two instances of the data? After you import into Apple Mail, you will want only one instance of the data on the external drive. It will be in your Thunderbird profile and that will be the only place where Thunderbird accesses it.

(I hope I did that above 'blockquote' correctly 😬) Regarding whether I expect to end up with "two instances of the data" ...it depends on our definition of "data." Hoping this clears it up: I'm expecting to retain Apple Mail's management of the Gmail account as I've had it for the past 10 yrs or so – synced with Gmail. <– that could be one set of data. Separate from that Apple Mail access, though, I want to be able to access and search the historical emails (downloaded as an mbox from Google Takeout) without affecting what's in Gmail. <– that could be considered another set of data. Hope that makes sense (?) • Also to clarify, I won't be importing any data into Apple Mail.

Thunderbird never needs to know about any external e-mail accounts, including gmail.

This is helpful and reassuring! Thank you. I don't want to be messing with emails in Thunderbird and accidentally have those changes propagate in Gmail.

And finally, although I am basically fine with installing Thunderbird on the Mac, I'd prefer if both the Thunderbird profile data and the app itself were on the USB drive. It would feel cleaner: 100% of my backup email archive, Thunderbird itself included, is on that drive. But if achieving that involves more than just dragging the app to the USB drive during installation, I'm not up for it. 😂 ...so I was just wondering.

Thanks again so much for all your replies, @Rick and @Michel T

Thanks so much for your responses!

@Rick, • I've never used Thunderbird before 😬 • but yes, I did find this support article Profile Manager - Create and remove Thunderbird profiles while researching solutions and together with your response, it clarifies things for me. Thanks!

how do I make sure TB knows that the data from the mbox(es) I import should "live" on the external drive as well?

I'm not sure that I understand. Are you expecting to end up with two instances of the data? After you import into Apple Mail, you will want only one instance of the data on the external drive. It will be in your Thunderbird profile and that will be the only place where Thunderbird accesses it.

Regarding expecting to end up with "two instances of the data"...it depends on how we define "data." I'm expecting to retain Apple Mail's management of Gmail so Apple Mail can be used to delete/reorganize emails. <– that could be considered one set of data. And I'm also expecting to be able to access and search historical Gmail emails via Thunderbird. <– that could be considered another set of the same data. Hope that makes sense (?) • To clarify, I'm not going to import anything into Apple Mail; I tried that and it failed, so am turning to Thunderbird.

Thunderbird never needs to know about any external e-mail accounts, including gmail.

That's helpful/reassuring, thank you! I don't want to work in Thunderbird and accidentally be altering what's in Gmail.

Finally, although I can basically be fine having Thunderbird software live on the Mac while the Thunderbird profile data lives on the USB drive, I'd prefer to have both stay on the USB drive. Only because it feels cleaner that way: 100% of the email backup is on the drive and portable. But, if achieving that involves more than simply dragging Thunderbird to the USB drive during installation...I don't want to deal with it. 😂 Just wondering if it's that's easy? And again, if not, or if maybe not, or we don't know...I don't want to deal with it. So, just wondering.

Thanks again @Rick and @Michel T for your help!

I think that I understand everything now.

I've used Macs for almost forty years, but have never installed apps outside the application folder. Whether or not Thunderbird can be installed on an external drive probably depends on the components that Thunderbird installs in different places of the local drive. See the attached screenpic. Caches and plist may be particularly important. I don't know if it is possible to make Thunderbird access such things if it is not installed in the Applications folder.

You have probably already read: https://support.mozilla.org/en-US/kb/installing-thunderbird-on-mac

Importing into Apple Mail may have failed because you tried to import everything at once. If you requested individual mboxes in your latest Takeout request, you may be able to import those smaller mboxes into Apple Mail.

Thank you @Rick! Makes sense. And thank you @Michel T. I too, might be a billionaire if I had a dollar for every misunderstanding 😜