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6.3.6. Activating and Mounting the Original Logical Volume

Since you had to deactivate the logical volume mylv, you need to activate it again before you can mount it.
# lvchange -a y /dev/myvg/mylv

[root@tng3-1 ~]# mount /dev/myvg/mylv /mnt
[root@tng3-1 ~]# df
Filesystem           1K-blocks      Used Available Use% Mounted on
/dev/yourvg/yourlv    24507776        32  24507744   1% /mnt
/dev/myvg/mylv        24507776        32  24507744   1% /mnt