This article is from the Macintosh hardware FAQ, by Elliotte Rusty Harold firstname.lastname@example.org with numerous contributions by others.
Yes, but I certainly wouldn't trust any data I intended to keep
for more than the next minute or two to such a diskette. The extra
hole is not the only difference between a double-density and a
high-density disk. The magnetic media on a high-density disk
is a different type that is much more susceptible to formatting.
Double-density diskettes should only be formatted as 800K. With the
plummeting prices of real high-density disks, this really shouldn't
be an issue anymore.