Answer by Kamil Maciorowski for Is it possible to combine two USB flash drives into one single partition to store a very large file, and if so, how can this be achieved?

If "logical partition that would appear as one larger disk" is not a strict requirement then you can use Btrfs.

First decide if you want to commit whole USB sticks to the filesystem you are going to create, or if you want to create a partition table on each. The latter method is more canonical, the former method is simpler.

Most commands in this answer need root access. For brevity I do not include sudo explicitly in the commands. Use sudo where needed.

Let's assume the devices you want to use are /dev/sdx and /dev/sdy. Just in case run wipefs -a /dev/sdx /dev/sdy, so the devices appear clean.

If you want to use partitions rather than whole devices then create one big partition per device (with fdisk, gdisk or a similar tool). If your tool of choice asks you if it should create a new filesystem, decline. If the tool finds an old signature and asks you if it should erase it, affirm. After creating partitions (say /dev/sdx1 and /dev/sdy1), just in case you may run wipefs -a /dev/sdx1 /dev/sdy1, so they appear clean for sure.

Now create a Btrfs on the devices. It will be either

mkfs.btrfs -d single -m dup /dev/sdx /dev/sdy

or

mkfs.btrfs -d single -m dup /dev/sdx1 /dev/sdy1

depending on if you want to use the whole devices or the partitions.

This will not create a "logical partition that would appear as one larger disk"; I mean there won't appear any /dev/something you can access on the block level. Still, a larger filesystem will be created and ready to be mounted.

To mount the filesystem, mount any of its devices, no matter which one. If the kernel knows which other devices belong to the same filesystem then it will use them properly. If any of the devices is missing (really missing or "missed" by the kernel) then the kernel will not let you mount the filesystem. Just after creating the filesystem the kernel should know the devices and mounting should work straightforwardly. After a reboot or after (re)connecting the device(s) to the same or another Linux you may need to run btrfs device scan first, to make the kernel examine all devices and learn what Btrfs is where. There is no harm in running btrfs device scan even if the kernel already knows; so when in doubt, just run it.

This is how you mount:

mount /dev/sd… /path/to/mountpoint

where sd… is one of the devices that belong to the filesystem. In our example it can be sdx or sdy (if the filesystem is on these whole devices), or sdx1 or sdy1 (if the filesystem is inside these partitions).

You unmount in the most regular way:

umount /path/to/mountpoint

Notes:

• To store even larger files, you can use three or more devices with mkfs.btrfs in the first place; or you can add a device to Btrfs later with btrfs device add … (see man 8 btrfs-device).

• You can use whole devices, partitions and/or even regular files* as "devices" committed to Btrfs. You can mix these types.

  ^{* In case of regular files, if you need to run btrfs device scan then you should also run btrfs device scan /regular/file1 /regular/file2 … because by default the tool scans block devices only, not regular files, so you need to explicitly inform the kernel of the right regular files to scan.}

• The comment that says "USB flash drives are notoriously unreliable and you'll be doubling the chance something goes wrong" is basically right. I'm giving you a way to do what you want because it's possible. Personally I would use Btrfs on multiple USB flash drives only for short-term (ad hoc) storage of expendable data.