If WiiFlow can't boot either then forcing IOS58 might not work. But you can try for yourself by opening up meta.xml in notepad, removing the two lines to enable arguments and then changing
--ios=249 to
--ios=58. Or you can go the other route and keep the IOS set to 249 but change the boot IOS from 58 to 249.
I don't know if it'd work, but NTFS might also be an option.
I know that Priiloader doesn't pass arguments stored in meta.xml to the loader correctly, so depending on your settings maybe that's got something to do with it.
If you've got the loaders UNEO channel forwarder installed then you can download
this zip file and extract uneoboot.dol to the root of your SD card. Then boot to Priiloader and install that DOL file instead.
I'd also recommend using
Priiloader 0.10.0-beta2.