I would like to convert an ext4 partition to ext3 partition without formatting the HDD.
Is this possible and recommended?
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From the Ext4 FAQ and Ext4 Howto, the short answer is "no". The longer answer is that if you created the ext4 filesystem from an existing ext3 filesystem but didn't enable certain ext4 features such as extents, you would still be able to mount it as ext3. However, if the ext4 filesystem was created from scratch you almost certainly used new features of ext4 that prevent you from treating it as ext3.
All ext3 partitions can be mounted as ext4. But not all ext4 partitions can be mounted as ext3. Some can. It's backwards compatible in that you can mount ext2 and ext3 partitions as ext4 and they are perfectly valid ext4 partitions. It's forwards-compatible in certain circumstances in that you can mount some ext4 partitions as ext2 or ext3.
ext4 is a set of extensions to the ext2 and ext3 filesystems. It adds optional extensions to that filesystem but it includes all the features and extensions of ext2 and ext3.
Some of these optional extensions are not on-disk, so they don't affect your data or your ability to continue mounting the filesystem as ext3. But others, particularly extents, are ext4-only and once you have used this feature of an ext4 partition there is no going back to being able to mount it as ext3 without re-formatting.
If you formatted an ext4 partition from scratch then it probably uses extents and therefore you cannot mount it as ext3 without re-formatting. If you originally formatted as ext3 but have been mounting it as ext4 then you can safely go back to mounting it as ext3. If you did a "conversion" when you moved to ext4 then chances are you've turned on ext4-only on-disk features and therefore cannot go back to mounting it as ext3 without re-formatting.
If you try and mount an ext4 partition as ext3 but it is not compatible, it'll just refuse to mount it - so you don't need to worry about screwing up your drive if you get it wrong.