A partition copied with cat can't be mounted
cat is not an appropriate tool for copying volumes. It dumps files character by character. Yes, unix abstracts everything to a file, but it also distinguishes between character devices (files) and block devices (storage).

Wipe the partition and use dd, rsync, or a GUI tool like Carbon Copy Cloner to make a duplicate.

However, if I read your sequence correctly it looks like you made a copy and then wiped the original without verifying your copy. If so, time to get the backups out.