Correct, XFS filesystems cannot be shrunk (reduced) dynamically, like ext2/3/4 can, only grown. You need to create another, smaller filesystem (XFS too usually) then copy the existing content which can be done with tools like xfsdump & xfsrestore (iff both are XFS), rsync, pax (spax), or tar, typically while the system is in single user mode so that no changes can occur (and be lost). Which exact steps depend, as usual. If there's nothing on that USB drive you want then you can expand the VG onto it (gdisk, pvcreate then vgextend), make a new, smaller LV (lvcreate) and filesystem (mkfs), copy the content, remove the old LV (lvremove), then move the new LV off the USB array onto your normal storage (pvmove) then remove the USB array from the VG (vgreduce), at which point you should be able to grow (lvextend) your root. /mark