Sony A7RIII ILCE-7RM3A Operating Instructions Manual page 154

When you use a memory card with the camera for the first time, it is recommended that you format the card on the camera for
more stable performance of the memory card.
You can change the memory card slot for recording by selecting MENU →
Rec. Media].
To record the same image on two memory cards at the same time or to sort recorded images onto two memory card slots by the
image type (still image/movie), select MENU →
Use the slot 1 when you are only using one SD card.
Use the slot 2 when you are using Memory Stick media. In this case, set [Prioritize Rec. Media] to [Slot 2].
Do not insert a Memory Stick media into the slot 1. It may cause a malfunction.
To remove the memory card
Open the memory card cover and make sure that the access lamp (A) is not lit, then push in the memory card lightly
once to remove it.
Related Topic
Memory cards that can be used
Notes on memory card
Format
Rec. Media Settings: Prioritize Rec. Media
Selecting which memory card to play back (Select PB Media)
Rec. Media Settings: Recording Mode
(Setup) → [Rec. Media Settings] → [Recording Mode].
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(Setup) → [Rec. Media Settings] → [Prioritize