Sorry false alarm. The initrd I use to test is from arm64 live ISO and it is compressed by zstd which not installed. With zstd installed unmkinitramfs works fine too.
** Changed in: initramfs-tools (Ubuntu) Status: New => Invalid -- You received this bug notification because you are a member of Ubuntu Touch seeded packages, which is subscribed to initramfs-tools in Ubuntu. https://bugs.launchpad.net/bugs/1953297 Title: unmkinitramfs does not work on arm64 Status in initramfs-tools package in Ubuntu: Invalid Bug description: unmkinitramfs in initramfs-tools-core works fine on amd64 but same command fails on arm64 On amd64: $ unmkinitramfs initrd temp $ ls temp early early2 main On arm64: $ unmkinitramfs initrd temp cpio: premature end of archive $ ls temp $ The tested unmkinitramfs is 0.136ubuntu6.6 in focal-update archive To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/initramfs-tools/+bug/1953297/+subscriptions -- Mailing list: https://launchpad.net/~touch-packages Post to : touch-packages@lists.launchpad.net Unsubscribe : https://launchpad.net/~touch-packages More help : https://help.launchpad.net/ListHelp