I didn't make it!
Throughout Clear Sky, I have obtained five flash drives. I'm up to the part where I first get to the Duty base left of the Garbage area. I'm talking with Thunderov and he's asking me to bring him some flash drives, all of which I have. However, when I try to talk to him, there is no conversation option other than "Alright, bye," and there is an upgrade button. When I press the upgrade button, it tells me to upgrade a weapon. I was hoping he would be rewarding me or something over the fact that I had brought him the flash drives, since he specified a certainly large amount of money that he would give me. I don't have any money because the bandits on the edge of the Garbage took it all from me and it's actually charging me to upgrade my weapons where I want to give him the flash drives. How do I give Thunderov my flash drives so he'll pay me?
I have had that problem from time to time. The game is so poorly written, that when you can't give them the flash drives, you are stuck with them.
Odd. I have heard of the trouble with Freedom mechanic where if you take a particular mission before turning in all the flash drives he asks for, you will never be able to due to a bug in the dialogue tree. There is a mod fix for that. However, I never had it happen with the Duty mechanic.
I assume you have the flash drives that he specifically wants? Each mechanic requests different drives and they must be returned to that mechanic and no one else. If you do have the right ones and he won't take them, you may be out of luck. :(
There is mod that makes the Clear Sky faction mechanic able to upgrade everything without the need for flash drives, though. It's not exactly a fix, and it won't help your lack of cash to actually purchase the upgrades, but at least you would have those advanced upgrades available to you. I would link you to the file at STALKERFiles but the site is FUBAR right now and it would be pointless. :P It is called "CS Mechanic Upgrades Everything," for future reference.