How to permanently archive Time Machine backup?
I would recommend using hdiutil to image the disk.

First, run diskutil list in the Terminal to see the identifiers for each disk. You'll want to note the identifier for your Time Machine drive (format is /dev/disk#).

Then, in the Terminal, use:

/usr/bin/hdiutil create -srcdevice /your/disk/identifier -format UDZO name.dmg


Where /your/disk/identifier is the /dev/disk# corresponding to your Time Machine drive, and name.dmg is the name of the output DMG. The UDZO format is the default compressed disk image format. The disk image will be a replica of the entire USB drive.