Flash-Drive problems

Try repairing it:

Repair a Hard Drive

Boot from your OS X Installer disc. After the installer loads select your language and click on the Continue button. When the menu bar appears select Disk Utility from the Utilities menu. After DU loads select your hard drive entry (mfgr.'s ID and drive size) from the the left side list. In the DU status area you will see an entry for the S.M.A.R.T. status of the hard drive. If it does not say "Verified" then the hard drive is failing or failed. (SMART status is not reported on external Firewire or USB drives.) If the drive is "Verified" then select your OS X volume from the list on the left (sub-entry below the drive entry,) click on the First Aid tab, then click on the Repair Disk button. If DU reports any errors that have been fixed, then re-run Repair Disk until no errors are reported. If no errors are reported, then quit DU and return to the installer.

If DU reports errors it cannot fix, then you will need Disk Warrior and/or Tech Tool Pro to repair the drive. If you don't have either of them or if neither of them can fix the drive, then you will need to reformat the drive and reinstall OS X.

Sorry, I don't have an answer for you, but a question that's not related to your topic but just an issue I'm curious about because I'm having trouble with this.

Do you find it difficult to plug your flash drive into your iMac? What port do you use - rear or keyboard?

About your flash drive itself - I had a similar issue with a Sandisk drive that was many years old. I did what Kappy suggests above but Repair in DU didn't help. Only reformating made it useful again (fortunately I had duplicate copies of the data on it, hopefully you also have a copy of the work on your drive).

Mrs H