Mine will play MP3s on a 2 GB card but not a 4 GB card. It seems like it takes about 20-30 seconds to read the card and start playing.
I initially separated the MP3s into separate folders based on genre like I do for the Pioneer head unit in my truck, but found the ZX75 could only read the files if they weren't in folders.
Hope that helps.
2011 Rockwood Signature Ultra Lite 8293SS, 12K Equal-i-zer
2016 Ford F-250 XLT SuperCrew, 6.2L, 4x4, 6'9" bed, 3.73
2010 Rockwood Roo 23SS (2012 - 2014)
2009 Ford F-150 XLT SuperCrew, 5.4L, 4x4, 5'6" bed (2012 - 2016)