My understanding is that the PCIe expansion is being used for the HAT+ only works for one disk peripheral if it's being used for booting the OS.
For example, since for HAT+ you can have a maximum 2 stacks of HAT+ boards expansion. Let's say you have one M.2 HAT+ and another is GbE board available from PineBerry [1]. The new additional GbE port on the HAT+ (not the original on-board GbE on the RPi 5) will interfere with the M.2 disk on the HAT+ if it's being used for booting.
[1] PineBerry Pi launches five additional HAT+ boards for the Raspberry Pi 5 with 2.5GbE, GbE, M.2 NVMe, and more:
For example, since for HAT+ you can have a maximum 2 stacks of HAT+ boards expansion. Let's say you have one M.2 HAT+ and another is GbE board available from PineBerry [1]. The new additional GbE port on the HAT+ (not the original on-board GbE on the RPi 5) will interfere with the M.2 disk on the HAT+ if it's being used for booting.
[1] PineBerry Pi launches five additional HAT+ boards for the Raspberry Pi 5 with 2.5GbE, GbE, M.2 NVMe, and more:
https://www.cnx-software.com/2024/03/08/pineberry-pi-hat-plu...