It’s always possible, but in my experience, the magnets in those graphics card stands aren’t strong enough to hurt the data in a hard drive slotted below them.

If you’re concerned, you could always replace the magnet with a rubber foot, or replace the whole stand with nonmagnetic cylinder like a paper towel spool. The graphics card’s weight will probably hold it in place.

In any case, I recommend regularly making incremental backups of your data. Hard drives eventually die even when no magnets are around.