USB flash drives not recognized on my iMac for backup

Ok, thanks I now understand what you are trying to accomplish.

A few things to note, when using drives for Time Machine (TM) backups:

• An external drive must be formatted in either HFS+ or APFS.
• Whatever drive you use should be 2 to 3x the storage size of the internal Mac's drive. So, for example, if your iMac has a 1TB internal drive, you would need a 2 or 3TB drive for these backups.
• Flash drives are really not designed for this purpose, regardless of their storage capacities. Using them for TM will "wear" them out quickly as they have a limit to the number of data writes they can take.
• For TM, you will want get an external HDD. These come in much higher capacities than SSD/flash drives, and come at a much lower price point. Examples, would be the WD My Passport or OWC Express.

On the other hand, if you only want to copy a number of files on them, they should be fine for that purpose.

Ref: Backup disks you can use with Time Machine - Apple Support

Two very important bits of information is missing in your post:

1. What type of backups are you referring to? Time Machine or another third-party backup app?
2. What are the makes & models of these "USB flash drives?"

Ref: Writing an effective Apple Support Communities question

"onn" = ? Sorry, I have no idea what that means.

Also, what are the answers to my other questions?