Why not

  cd /mnt/target
  lzma d /media/cdrom/boot/rootfs.gz -so | cpio -id

??

I thought I had tried that and found that it failed trying to write a file rootfs.lzma onto the ro /media/cdrom/boot.

But I just tried again to check  -  and your suggestion works  (with /dev/shm instead of /mnt/target).   I must have made some mistake at the time.     Thanks for pointing it out.

Anyway, a question -  why is rootfs.gz named rootfs.gz and not rootfs.lzma?

John

You do not write to /mnt/cdrom/boot/, you read from it
Why shouldn't the system read from a ro media?

If I read you r message right, you write directly onto the device (/dev/shm) ans NOT to the mountpoint (e.g. /mnt/target) of that device?

As for the name, I really don't know.

Rgds.

I didn't explain very well.

*I* do not want to write to /media/cdrom/boot.     In my earlier test last Saturday, I tried what you suggested (since it's the most obvious) and it did fail,  and my quick diagnosis was that *it* (the SliTaz version of lzma) had been trying to do some kind of unpacking or transformation of the rootfs.gz in that directory where that file was located,   i.e. /media/cdrom/boot.    And I concluded that the task could not be done that way.   I formed the opinion that somehow it was sensitive to the suffix of the input file.   So I came up with the symlink idea,  which worked.

Today I tried again the same test except I did not want to repeat exactly as before because (if it had worked successfully)  that would have overwritten my root filesystem.     So I modified the test to write to /dev/shm instead of /mnt/target.    There should be no difference in result other than where the rootfs is unpacked to.   And this time it worked.

So I think I must have made some mistake on Saturday and your suggestion should work fine.

I hope that makes it clear?

John