I'm thinking about maintaining a full backup of the entire blockchain on an HDD, in case I ever need to set up a new node, so that I don't have to do the IBD again.
But why? Taken slightly out of context, it reminds me of this:
“Only wimps use tape backup. REAL men just upload their important stuff on ftp and let the rest of the world mirror it.”
Unless you're restricted on bandwidth or RAM, I wouldn't worry about backup up blockchain data as long as thousands of nodes have a backup online!
3. make periodical update backups (I don't know how often).
~
(b) What's the ideal process?
I would use this:
rsync - a fast, versatile, remote (and local) file-copying tool
--link-dest=DIR hardlink to files in DIR when unchanged
This way, you don't need to overwrite old backups, while keeping "snapshots" of older backups barely takes more disk space than your latest backup.