Was the flash drive previously working with your iPad and iPhone? If so, do you recall the version of iOS/iPadOS with which you were previously using this drive?
What happens now? Do you see any reported errors? What do you see?
Connection of external storage devices is possible - with some caveats...
To be recognised by iOS/iPadOS, the attached USB storage device must be formatted to FAT, FAT32, exFAT or APFS with a single partition. As the iPad has limited power capabilities, dependent upon the Storage Device, an external power source is often required.
If your iPad or iPhone has a Lightning port, you will need to use an Apple Lightning to USB3 Camera Adapter to directly connect USB devices to the device. This adapter has both a USB 3.0 Type-A connector and an additional Lightning port to supply power, from your iPad Power Adapter, to both the iPad and connected device. The Power Adapter must be rated at 12W or more.
If you have one of the newest iPad models with a USB-C connector, you may need to utilise a powered USB-C hub (i.e., it has its own source of power) - connecting both the iPad and the storage device to ports on the hub.
Some additional information can be found here:
Connect external devices or servers with Files on iPhone - Apple Support
https://www.imore.com/how-use-external-hard-drives-your-iphone-or-ipad