They were all flexible, but the PC isn't. :) But what you are referring
to, is that from TOS 1.62 (ie from the STe on), disks formatted from the
Desktop are DOS-compatible.
However, any replacement formatting tool (FastCopy, Kobold etc) for the
ST will do the job.
On the other hand, you can also use GemExplorer on the PC to read
pre-1.62-Atari-formatted disks, but apparantly that doesn't (yet?) run
And then there are other cool options, like connecting ST to PC with a
nullmodem cable and transferring the files using a terminal application
or setting up a PPP link between them and use ftp. :)