my backup hard drive has an invalid partition table

Since I copied a partition to my backup hard drive with dd earlier today, Gparted has started displaying an "invalid partition table" error message for it when it scans my media. The error makes gparted frequently unable to accurately read the drive. I refresh, and sometimes I can see the partitions--they appear to be still there--but often it reads the drive as blank.

What's the quickest way to fix an invalid partition table? The only thing I know I can do, if it even works under the circumstances, is dd the partitions individually to another medium and then re-create the partition table, eliminating the error. (I can't do that now, of course, because it would erase everything. I want to correct the partition table, not destroy it.)
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