partimage to backup my
ext4 partition, but during backup, the partition was detected as an
ext3 partition. So I'm wondering if this can cause something bad.
Limitations - Partimage does not support ext4 or btrfs filesystems.
It is unwise to use it for ext4 as long as that message is on their website.
If you don't use extents, ext4 can be mounted as ext3.
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Use fsarchiver instead. fsarchiver supports ext4.
It can be thought of as a successor to Partimage, which has mostly ceased development since 2009. It is written by one of the Partimage developers, Francois Dupoux, and has other features which Partimage lacks. See the comparison table FSARCHIVER VS PARTIMAGE.