Because as you suspect, an image backup cannot be done while the partition being imaged is live.

can’t it, though?

macrium reflect’s normal operation is to run when the ststem is running normally. it creates a volume shadowcopy of your filesystem, and backs that up. a BTRFS/ZFS snapshot is basically what a volume shadowcopy is on windows, but with a less fancy name. if you make a snapshot, you can back that up, either with zfs send, btrfs send, rsync, borg backup, whatever. the difference is that on linux it’s not possible to notify programs that a snapshot will happen please sanitize your databases, while windows does that too, so if you restore on linux that’s like if your computer crashed because power went off

sure, it can’t be done with other filesystems, but OP said they have BTRFS. I think the boot partition can be safely imaged too: remount as read only and make a normal image.

it gained 14k+ stars on github in a year (development started in 2023 july).

isn’t it a bit suspicious?

maybe it’s nothing, but this just caught my eye