What are Dirty SD cards?
If an SM3 or SM4 series recorder detects a problem with an SD card, it will mark the card as "Dirty" and no longer use that card, to avoid corrupting any further data. This can happen if a recorder momentarily loses power in the middle of writing a file. When it wakes back up and sees a half-written audio file, it suspects that the SD card may be corrupted, so it will mark the card as "Dirty."
Dirty cards can be used again after they are formatted, either using the Format Cards utility on the SM3 or SM4, or by using the SD Association Formatter.