Hey zuiko3000,
Based on the information you've provided, it sounds like other drives are working for you but the SanDisk drive isn't. As the issue is specific to that drive, we'd suggest contacting SanDisk support to see if they have further information for you on that.
SanDisk Support
Prior to contacting them, take a look at this page about using external drives with an iPhone, as it has there format types and other info:
Connect external storage devices to iPhone - Apple Support
Connect a USB drive or an SD card reader
1. Attach the USB drive or SD card reader to the charging port on your iPhone using a compatible connector or adapter.
You may need the Lightning to USB Camera Adapter, Lightning to USB 3 Camera Adapter, or Lightning to SD Card Camera Reader (all sold separately).
Note: A USB drive must have only a single data partition, and it must be formatted as FAT, FAT32, exFAT (FAT64), or APFS. To change the formatting of a USB drive, use a Mac or PC.
2. Do any of the following:
• Insert an SD memory card into a card reader: Don’t force the card into the slot on the reader; it fits only one way.
Note: You can import photos and videos from the memory card directly to the Photos app. See Import photos and videos on iPhone.
• View the contents of the drive or memory card: In a supported app (for example, Files), tap Browse at the bottom of the screen, then tap the name of the device below Locations. If you don’t see Locations, tap Browse again at the bottom of the screen.
• Disconnect the drive or card reader: Remove it from the charging port on iPhone.
Note: The Lightning to USB 3 Camera Adapter can be powered with a USB power adapter. This allows you to connect USB devices with higher power requirements, such as external hard drives, to iPhone.
Thanks!
