Problem opening ODK after downloading

We encountered an issue loading ODK on a health promoter’s phones yesterday. All of the health promoters have the same phones – but the others were able to download and open ODK without any problems. All phones have a 4GB SD card. We received the following error message on just one of the phones after downloading ODK and attempting to open:

ODK reports::cannot create directory:/mnt/sdcard/odk/cache

Any suggestions for how to fix this? We already tried swapping out SD cards, but that didn't fix the problem.