Why does Timeshift put up this message when I try to backup to an external hard drive? Deja Dup, Lucky and Pika don't
Because Timeshift doesn't simply copy files around from one location to another. The way it works is that it keeps track of changes made to a file between each snapshot taken, thus is able to reconstruct the filesystem using that information.
The EXT4 filesystem, used by many Linux distribution, has certain metadata that are unknown to other types. Things like file permissions, for example. Saving files to an external drive with another filesystem that doesn't understand this metadata would simply remove it. This would hinder Timeshift's ability to reconstruct the snapshot accurately, and so you can only choose filesystems natively supported by Linux.
Other backup tools basically copy the files, meaning its contents, or may be more permissive and shift the responsibility to deal with this to the end user.
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It's worth noting that some other backup tools also offer sector by sector backup, at a level lower than the filesystem, in which case such metadata isn't lost, as it's not backing up files; it's backing up the literal contents of the disk. (I'm not saying this to dispute you in any way; only to point out that no message doesn't necessarily mean lost metadata/user responsibility.)
So does this mean I should be formatting everything as NTFS instead of ext4?
If I understand well what @zenzen said, you need to have an external drive formatted in EXT4 (or a partition) to have a complete snapshot
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If you are going to use Timeshift on Zorin and use an external drive you must either use the whole drive as Ext4 or if dual-booting create separate partitions formatted to Ext4 for Timeshift snapshots, and another formatted to NTFS for Windows files. Because I use 1Tb HDDs I have my Timeshift snapshots stored in /home. If I am backing up my /home folder I have had no issues copying same to a preformatted NTFS drive.
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I've formatted a whole USB stick as ext4, but Timeshift still says it needs a Linux partition, but I don't know how to do this
Lets say /dev/sdb1 is the path to the external drive (adjust accordingly to your path with lsblk | grep sdb or check gparted)
You can use
sudo umount -lf /dev/sdb1
mkfs.ext4 /dev/sdb1
to achieve that.
After this you are able to select your drive in Timeshift.
This is what I get
tom@OldMesh:~$ sudo umount -lf /dev/sdc1
mkfs.ext4 /dev/sdc1
[sudo] password for tom:
umount: /dev/sdc1: not mounted.
mke2fs 1.46.5 (30-Dec-2021)
mkfs.ext4: Permission denied while trying to determine filesystem size
tom@OldMesh:~$
sorry... you need use sudo
type command after command
Correct Command:
sudo umount -lf /dev/sdc1
sudo mkfs.ext4 /dev/sdc1
tom@OldMesh:~$ sudo umount -lf /dev/sdc1
[sudo] password for tom:
umount: /dev/sdc1: no mount point specified.
tom@OldMesh:~$ sudo mkfs.ext4 /dev/sdc1
mke2fs 1.46.5 (30-Dec-2021)
The file /dev/sdc1 does not exist and no size was specified.
tom@OldMesh:~$
lsblk | grep sd
So I've now got a USB drive formatted as Ext4 and, surprisingly, have got Timeshift to finally back up to it. Now, I have both a PC and a laptop with Zorin and want to use Timeshift to backup both to the same ext4 USB drive. How do i make directories on the USB drive?
Edit. OK, I've done that using ```
mount /dev/sdc /mnt
cd /mnt
But all to no avail, as it seems Timeshift always saves to /timeshift, so I cant have two Timeshift backups on one drive 
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