How to Split a 20GB File into Parts That Fit on a FAT32 Drive (and Rejoin Them)

You need to move a 20GB file — a long video, a disk image, a game backup — using a drive formatted as FAT32. The copy fails every time, because FAT32 cannot store any single file larger than 4GB. Reformatting the drive is not always an option, especially if it belongs to someone else or an older device demands FAT32.

The classic workaround is to split the file into pieces, each small enough for FAT32, copy the pieces across, and rejoin them at the destination into the exact original file. Done correctly, the rejoined file is bit-for-bit identical to the one you started with.

Quick Answer

Install the free 7-Zip, right-click your 20GB file, and choose Add to archive. Set Split to volumes to about 3,900MB, which creates roughly six parts named .001, .002 and so on. Copy all parts to the FAT32 drive, then at the destination put every part in one folder and extract starting from the .001 file — 7-Zip rejoins them automatically into the original file. Verify with a checksum if the file matters.

Why the Parts Must Be Under 4GB

FAT32’s maximum file size is 4GB minus one byte — 4,294,967,295 bytes. Archiving tools measure in their own units, so choose a volume size with a safety margin. Setting 3,900MB keeps every part comfortably below the limit in both decimal and binary measurements, and avoids the classic failure where the last part is created at exactly the boundary and the drive rejects it. A 20GB file at 3,900MB per part produces six parts: five full ones and a smaller remainder.

Splitting the File with 7-Zip

1. Install 7-Zip from the official 7-Zip website. It is free and adds right-click options to File Explorer.
2. Right-click the 20GB file and choose 7-Zip, then Add to archive.
3. Set Archive format to zip or 7z. If the file is already compressed (a video, an ISO, an existing archive), set Compression level to Store. Re-compressing a video wastes an hour and saves almost nothing.
4. Set Split to volumes, bytes to 3900M.
5. Click OK and wait. With Store mode, splitting is mostly a read-and-write operation and finishes far faster than real compression.
6. Check the parts. You will see files like filename.7z.001 through filename.7z.006. Count them and note the total size roughly matches the original.

Copying and Rejoining at the Destination

1. Copy every part to the FAT32 drive. Missing even one part makes the whole set useless, so copy the folder, not individual files from memory.
2. At the destination PC, copy all parts into a single folder on a drive with at least 20GB free — you need room for the parts and the rejoined file at the same time.
3. Right-click the .001 file only and choose 7-Zip, then Extract here. 7-Zip automatically reads the following parts in sequence. Never start extracting from .002 or later; the archive header lives in the first part.
4. Wait for extraction to finish and confirm the rejoined file has the expected size.
5. Verify the result if the file is important, then delete the parts. If a copy to the FAT32 drive keeps stalling or failing partway, our guide on resuming a failed copy exactly where it stopped with Robocopy shows how to avoid starting over each time.

Rejoining Without 7-Zip

If the parts were created as plain split chunks rather than an archive — some tools produce name.part1, name.part2 style files with no compression — Windows can rejoin them from the command line. Put all parts in one folder, open Command Prompt there, and run copy /b file.part1+file.part2+file.part3 file-complete. The /b switch is essential: it tells Windows to treat the data as binary. This only works when the parts are raw sequential chunks; for 7-Zip or WinRAR volumes, always use the original tool to extract.

Common Mistakes to Avoid

• Setting parts to exactly 4095MB. Unit rounding can push a part a few bytes over the FAT32 limit. Stay at 3,900MB or below.
• Renaming the parts. The .001, .002 extensions and the shared base name are how the tool finds the sequence. Rename nothing.
• Extracting from the wrong part. Always start with .001. Starting anywhere else produces an error or a corrupt output.
• Deleting the original too early. Keep the source file until the rejoined copy is opened, played or verified successfully at the destination.
• Running out of space at the destination. You need the parts plus the full 20GB output simultaneously. Check free space before you start.

Verifying the Rejoined File

When the rejoined file matters — an archive of family video, a disk image, anything you cannot simply download again — prove it matches the original before deleting the source. Compare SHA-256 checksums: on the source PC, open PowerShell in the file’s folder and run Get-FileHash filename -Algorithm SHA256, then run the same command on the rejoined file at the destination. Identical hashes mean the split, the journey and the rejoin were flawless. If they differ, do not guess which part went bad by size alone; re-copy the parts whose sizes mismatch the source set, extract again, and re-check. The checksum turns a vague worry into a definite answer in under a minute.

Splitting Without 7-Zip, and on a Mac

WinRAR produces the same multi-part sets (choose the volume size in the archive dialog) and rejoins by opening the first part. On a Mac, the built-in command line can split any file with split -b 3900m bigfile.zip bigfile.zip., creating lettered chunks you rejoin with cat bigfile.zip.aa bigfile.zip.ab > bigfile.zip — or simply use the free Keka archiver, which offers split volumes in its interface. Linux users have both 7-Zip (as p7zip) and the same split/cat pair. Whichever tool splits, the discipline is identical: name the parts clearly, keep them together, record how many there should be, and verify the rejoined result.

Managing the Parts During the Journey

Six parts invite one specific accident: copying five of them. Before setting out, write down the part count and total size — a sticky note or a phone photo of the folder listing works. If the FAT32 drive will make several trips, put each file’s parts in their own clearly named folder (Film-Parts, Backup-Parts) so sets never mingle; two downloads’ worth of .001 files in one folder is a confusion that has ruined real afternoons. And if a part fails to copy mid-journey, note which one — re-copying a single 3.9GB part beats restarting the set.

One last habit makes split transfers stress-free in the long run: keep a tiny text file named ABOUT-THESE-PARTS.txt alongside the parts, noting the original filename, its size, how many parts there should be, and which tool made them. Six months later, when you rediscover a folder of mysterious .003 files, that note turns confusion into a two-minute rejoin instead of a guessing game.

Conclusion

Splitting turns an impossible copy into six easy ones. Use 7-Zip with Store compression and 3,900MB volumes, keep every part together and untouched, extract from .001, and verify the rejoined file before deleting anything. The method is old, unglamorous and completely reliable.

Frequently Asked Questions

How many parts will a 20GB file make?

At 3,900MB per volume, expect six parts — five full volumes and one smaller final part containing the remainder.

Can I split a file without installing anything?

Windows has no built-in split tool. 7-Zip is free and tiny; on a locked-down PC, the HJSplit portable tool also works without installation and rejoins with its own Join function.

Will the rejoined file be exactly the same as the original?

Yes. Splitting and rejoining is lossless — the output is bit-for-bit identical, which a SHA-256 checksum comparison will confirm.

The extraction says a volume is missing or corrupt. What now?

Compare each part’s size against the source set. One part was probably copied incompletely. Re-copy just that part and extract again — you do not need to redo the whole set.

Is there a way to avoid splitting altogether?

If the destination device supports exFAT or NTFS, reformatting the drive to exFAT removes the 4GB limit entirely. Splitting is only necessary when FAT32 is genuinely required.