First, some general remarks (can be useful for other readers)
Atari engineers created two brilliant solutions for external devices. One is SIO (serial) interface, the other is PBI (parallel) one. Atari XL OS supports both types of devices without any patches, hacks etc. Handler routines can be automatically loaded into RAM from the devices and then attached to the operating system procedures.
There are many HDD interfaces using this two standards:
SIO (slow) ? SIO2IDE, SIO2SD, SIO2USB
PBI (very fast) ? Black Box, MIO, MSC, KMK/JŻ HDD interface (the latter is quite common here in Poland).
Both SIO and PBI HDDs can be used with SDX without problems.
About MyIDE (AFAIK)
MyIDE is a simple hardware, not conforming the two Atari standards. In fact, I would call it a ?signal converter?, rather than an interface. The computer should be supplied with tricky routines before it can access MyIDE HDD. They are included in a patched OS (MyIDE OS), which can be:
a. permanently present by replacing the original OS chip,
b. loaded into the RAM area under the OS and then switched on as a new OS.
With the first option you can probably use MyIDE with SDX 4.39, but only if you redirect a drive to ?OS? (with MAP command). The second option is not possible with SDX as well as with other software like Turbo Basic XL you mentioned.
To add MyIDE support to SDX, one should write a special handler including the MyIDE routines (I don?t know if their sources are freely available). The main routine has to check the DCB parameters and then perform a MyIDE HDD operation. If you supply me with such a code, I will prepare the appropriate driver and include it in the SDX ROM.