Two separate 4 GB walls hit large MBOX files: the FAT32 filesystem cannot hold a file over 4 GB, and some import add-ons stop at 4 GB. The fix depends on which one you are hitting. Jump to your situation below or work through the methods in order.
Large MBOX files run into two unrelated 4 GB limits. The first is the FAT32 filesystem: it cannot store a single file larger than 4 GB, so moving a big MBOX onto a FAT32 USB stick or drive fails with “file too large for the destination file system”. The fix there is to reformat the destination to NTFS, exFAT or ext4, which have no practical 4 GB limit. The second is a tool limit: Thunderbird's ImportExportTools NG has a hard 4 GB ceiling on direct import and warns or crashes on bigger files. The fix there is to bypass the importer entirely, closing Thunderbird and dropping the MBOX file straight into the profile's Local Folders directory so Thunderbird picks it up in place, or splitting the file. Identify which wall you are hitting and apply the matching fix.
The first 4 GB wall is the filesystem. FAT32, still common on USB sticks and SD cards, was designed when files were small and cannot address a single file over 4 GB. So a 6 GB MBOX simply will not copy onto it, no matter how much free space the drive shows.
The second wall is the tool. ImportExportTools NG, the popular Thunderbird add-on, has a hard 4 GB limit on its direct import and will warn or quietly crash on larger files. That is not a filesystem issue, it is the add-on, so the answer is to skip the add-on and let Thunderbird read the file in place.
“There are two 4 GB walls and people mix them up. If the file will not even copy onto a USB stick, that is FAT32, which physically cannot hold a file over 4 gigabytes, so you reformat the stick to exFAT and it copies fine. If the file copies but the Thunderbird add-on chokes on import, that is the add-on's own 4 GB limit, and the trick is to skip it entirely, drop the MBOX straight into the Local Folders directory and let Thunderbird read it in place. Different walls, different fixes.”
Archive2026) and copy it into your profile's Mail/Local Folders/ directory.There are two different 4 GB limits, so identify which one you are hitting. If the file will not copy, the destination is FAT32, reformat it to exFAT or NTFS. If the Thunderbird add-on stops at 4 GB, skip it by dropping the renamed file straight into the Local Folders directory, which has no such limit.
If the file is on a healthy non-FAT32 drive and still will not import after dropping it into Local Folders, the MBOX may have a malformed message that stops Thunderbird indexing it. Split the file and import the halves to isolate the bad section, then skip or repair that part. You can also submit your error to us for a tailored fix.