How to save my photos on iXpand usb ?

Forgot to mention that downloading the files to your Mac OS hard drive (e.g. under the folder 'pictures') preserves the correct timestamp information. From there you can copy the files to your external drive. By downloading the files directly to the external drive seems to set a new time stamp. Very strange, but that's how it is.

If the iPhone has an Apple Lightning connection port, you can use a SanDisk iXpand flash drive to use as an external hard drive. They are available in various sizes.

You can also use that iXpand flash drive as a tool to save the photos to a real external hard drive afterwards.

iXpand Flash Drive are available at Apple Store and at Apple Authorized Service Providers.

iXpand flash drive app can be downloaded from the App Store.

You have already -> SanDisk iXpand-flashdrive Go -> SanDisk 64 GB iXpand Flash Drive Go - Apple

Then download the SanDisk iXpand Drive app from the App Store, see -> 

https://apps.apple.com/us/app/sandisk-ixpand-drive/id1079870681

I tried the iXpand flash drive and was very disappointed. Technically the backup option works, but only as long as your iPhone is connected to the iCloud. When you are travelling and currently offline, it doesn't work. Not even manually selected files. And if you are online, please be aware that the iXpand app messes up the filenames. In short: it's not a solution that works for me. 

What does work for me is the Image Capture app on Mac OS. Just plug in your iPhone to your laptop or iMac via an original lightning cable, launch the Image Capture App, select the iPhone under 'devices' and select your backup device under 'Import To'. You have to manually select the pictures in question (you can select a range) and click 'download'. It works pretty fine and is now my preferred solution since the iPlugmate app (HooToo device) isn't available in the App Store anymore.