I believe this is the original article for arch linux on n900:
http://talk.maemo.org/showthread.php?t=81892
It suggests an option of using latest version of archlinux arm, you might want to try it, instead of using already modified archlinux rootfs.
Also it suggests running "sync" command after extracting files to make sure all the data is actually written to disk and not in buffer.
Before you to try the other option, can you try extracting again and run the sync command.
Make sure that uimage exists in /mnt/boot
Then run u-boot-update-menu while p5 is not mounted.

I am not really sure how u-boot-update-menu accesses the image file location and if it needs to be mounted, while running the command.
So, just wondering if mounting that partition is locking access to uimage.