Apple - USB Flash drive not mounting OS X

Based on the answers you've given in the comments.

• The device doesn't show in Disk Utility
• It doesn't show in System Information under USB
• It works in other computers
• Other USB sticks work in this computer – Changing USB ports makes no difference
• The device is getting warm – It also doesn't work in another OS X system

I would suggest that there is an incompatibility between this USB stick and OS X, it could be there is some kind of damage to the sticks Firmware or a wider issue with the product that you've not been able to trace down other reports of.

Short of opening up USB Prober and understanding more about how USB communications happen, I would suggest the stick is defective and you try to get a replacement from the retailer.

This may be helpful to some people who have simpler versions of this problem. In DISK UTILITY app, change the View Mode of Sidebar from default "Show Only Volumes" to "Show All Devices". Then the formatting options become visible.

In my case I had a brand new Silicon Power 16 GB flash drive pre-formatted as FAT32 which showed up on my Mac Desktop; it showed in the Finder, it showed in the "Get Info" window and verified as FAT32, but it would not show up in Disk Utility sidebar. I wanted to reformat it to Mac OS Extended Journaled but Erase and Mount were greyed out. I checked the System Report, it showed up on USB bus as well.
I finally noticed Disk Utility's sidebar panel had a down-arrow next to the "View option" at the top of the toolbar. Once I clicked the arrow to change views, I saw my USB volume, which then I was able to erase and reformat.

This is only helpful for those having problems Erasing & reformatting in Disk Utility, yet otherwise the flash stick is showing up.