Transfer photos from iPhone 12 to SanDisk Portable SSD

Plug the Sandisk SSD into the iPhone's Lightning port, if necessary use an adapter or cable that can fit both the SSD and the iPhone.

For instance, if the SSD has a USB-C port:

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With the drive connected to the iPhone, open the files App, and make sure the SSD has been detected and is accessible there under Locations in the Browse tab. If that is correct, then open the Photos App, select the photos you want to transfer, tap on the share icon, tap on "Save to Files" (you may need to scroll down a bit to see the option) and navigate to the External drive (you may need to tap on the < icon at the top, if you are in the "On My iPhone" location or some folder within that.) Select a folder or create one in the external SSD and and then tap on Save to start the transfer.

Is the cable rated for data transfer? Or just power delivery? Is it an Apple cable? Or third party?

How is the SSD formatted? The iPhone will mount and read fat32, exFat, HFS+ and APFS formatted drives. It won't mount drives formatted for Windows as NTFS.

Also, some drives may take longer to appear than others. If after 20 to 30 seconds it does not show up, try restarting the iPhone with the SSD unplugged, and then plug it in again to retry.

Sandisk also has a free app in the Apple App Store to help facilitate the transfer.

Remember that if iCloud Photos is on and "Optimise this iPhone" is turned on, the copies on iphone may be low resolution. Downloading direct from iCloud may be a better option for a master archive.