I use a sector-editor to dump my CF-card to the PC as a file.
(and visa versa)
In altirra, you have to add this file to your myide-][ device as hardddisk.
Use the CHS settings you get from the Atari, not use the preset values.
Use a filename that includes the CHS values, easy to remember
See picture.
Do not forget to add the 512k myide-][.bin file as rom content for your myide-][ device...
Optionally, you can add the internal MyIDE-][ OS as BIOS, so you do not need to load the F2.
Compatibility-wise this is a great enhancement.
Oh, in altirra, best is to disable all patched then, MyBIOS does not like patched OS-emulation.