|What format type are you using on the card? |
My guess is that you are writing all the files to the root directory in an older FAT file system. There is a limit of 512 entries in the root directory which will be further reduced by using long file names.
The exact number of 150 is strange though and kind of throws a wrench in my thoughts.
Try removing some and then making some folders. Cut and paste existing files to the folders and see if that solves your problem.