When using a SD2IEC and a 1541 floppy drive together it is possible to have a device id conflict. The SD2IEC defaults to device#8. The 1541 floppy drive defaults to device#8 as well.
To change the device id on the SD2IEC, power on the C64 but not the 1541. Then open a connection to the SD2IEC
Then send a command to change the device controller ID.
The SD2IEC should now be using ID 9. To test, load the directory.
Now power on the 1541 floppy drive. It should startup and work as device#8 as normal.