I know how that should be done but it doesn't work.
I want to save a (rsync) snapshot of my system with Timeshift on an external drive. I want to save thus a "perfect" exemplary of my system in order to restore it later in case the running one gets bloated or dies.
Timeshift offers a choice between two types of snapshots: RSYNC and BTRFS. RSYNC snapshots, unlike BTRFS, can be saved on external drives, but only on a Linux-type partition, that is NOT on an NTFS partition.
All my external drives are NTFS. So, I need to shrink the NTFS partition of one that has free space and create an ext4 partition there.
When the drive is mounted and I start Gparted all seems well.
But when it is unmounted (Gparted can unmount it) an exclamation point marks the partition, the free space is not seen, the shrink slider is stuck because no free space seems available, and info contain a warning .
Also, at this point the "mount" option is grayed out in Gparted. If I restart Gparted while the drive is not mounted, Gparted cannot mount it.
I was able to create an ext3 partition in Windows 10 with EaseUS Partition Master Free (which allows ext2 and ext3, not ext4 support). First I checked the drive for errors and then shrunk the ntfs to make space for the ext3. Oddly, EPM only allowed shrinkage of c.20GB out of the c.45GB of free space, so a c.19GB ext3 partition was created.
But in Ubuntu nothing changes as far as the NTFS partition is concerned: it is accessible in the file manager, appears mounted in Gparted, can be un-mounted there, but cannot be re-mounted nor modified by Gparted.