Yes .hdf is the Amiga format for Virtual HD's. You can also use .vhd, but Amiga Forever doesn't use that format. (WinUAE does though)
It's perfectly safe to make an Image of the card, in fact, it's best to do it sooner that later so that you have a backup. It's also handy to make a backup of the backup for testing software in WinUAE too.
WinImage can write back the .hdf to the CF card.
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