hard drives are going to be slow af copying data to itself, or moving data to a different partition on it.

then you're also adding partition size manipulation to the mix, which will also be slow af when data has to be moved off the 'end' of partitions to 'make room' to enlarge or create another with a different fs.

your best option is to get another drive, even if it's also a hard drive instead of ssd. use that to move (copy, really, to preserve the original as a backup for the time being) all the data to that you want to preserve.

Buy another 8 or 4TB, format it as ext4, copy everything from the ntfs HD to ext4 HD, swap drive, that's it. Keep the old HD to do some backup, put it in a usb3 encolusre.

Since you really should be creating a backup of the data before doing such a conversion in the first place, the best (not necessarily the fastest, but definitely the safest) way would be to copy the data to another medium, and copy it back when the space has been formatted.

Files are files and filesystems are filesystems. You keep your files on filesystems.

NTFS and ext4 are non convertible - you cannot turn one into the other directly, in place. However you can take files from one and put them on another.

Yes, moving TBs does take time, sorry it is unbearable.

Backup and restore after formatting? I would assume you have backups in place already

you can't get there from here.

you can convert to btrfs, but no matter what anyone says, there lie dragons.

if you end up doing big bulk reads and writes to and from your disks, keep them cool. if that means limiting bitrates with the rsync command or just plugging them up to a 802.11g laptop and copying from there to your target then that's fine. if it means making a funnel to direct a box fan's worth of cfm onto your drives then that's fine.

It'll be much faster the next time. It has to make sure all the data is out of the space to be freed. Assuming it moves it as close to the start of the partition as it can (and you're shrinking it from the end) then it'll be faster.

If you're shrinking it from the start, yeah, it's going to take forever because it will always have to move a lot of data.

If it is not sensitive data, and you're okay being morally objectionable, you can buy a large hard drive from some place with a good return policy, transfer your data to it, format/repartition your drive, transfer everything back and return the hdd for a 15% stocking fee.

I don't know anything that can do an in-place ext4 conversation, but there's ntfs2btrfs which is already in the Debian repos if you're okay with BTRFS.

Of course, backup anything important, ntfs2btrfs should create a backup snapshot if you need to revert back to NTFS, but I wouldn't count on it.

Use rsync -avs to move files from NTFS to your new filesystem. XFS is pretty good for lots of files.

Why not just leave them as NTFS for now? The new in-kernel NTFS3 driver is actually pretty decent (since kernel 6.2), and shouldn't pose any issues if you're just using it as a bulk data store.

Eventually when you replace the disks, you can can format your new disks as ext4 (or even better, use btrfs or bcachefs).