it won't be a problem for hard disks or usb external hard disks
maybe for crt monitors, could cause purple or green spots but the magnet itself can fix them again
a floppy possibly will end wiped, but since floppy discs should never existed and are a thing of the past, well, not much worries there
if you still use minidv cassete, that could be a problem
Shows how old I am.
I used to work in a room FULL of these damn things.
Mounting tapes all day long. Every one had a number from 1 to 18,000 and I had to know where everyone of them lived.
(Of course they were in numerical order so it wasn't that difficult to figure out)
Programs ran that would require data off of one and it would "request" the data be mounted on a specific drive. I had to supply that program with the tape it was looking for.
On occasion a program would have to "Write" data onto a blank (scratch tape).
We used a hand held electromagnet to "Scratch" a tape.
We wiped it all over the side of the reel to destroy all the data making it a "scratch".