how to reformat usb flash drive after formatting it in mac

You can try either or both of these methods.

On a Windows PC open Disk Management and select the USB drive. Change the partition structure from GUID/GPT to MBR by right clicking on the Disk # on the left. Once it is back to MBR you can then format it in a windows format.

On a Mac open Disk Utility and go to the partition tab. Select Once partition from the Partition layout drop down and then click the Options button and change that from GUID to MBR. Once that is done close the Options screen and then select a Windwos format, FAT, and click Apply.

If neither of those thing work then your USB thumb has failed. Buy a new one.

Them do it on a Mac with Disk Utility.

On Windows you might have to first delete all the partitions before you can select Convert to MBR.

But try it on OS X.

Open Disk Utility with the drive connected and select the Top name of that drive in the left most area of the window.

Then in the Partition Layout drop down select 1 Partition. Then the Options... button will become active, select that button.

Select Master Boot Record and click OK. The format should already be MS-DOS (FAT) but if it isn't select that. You can give it a name or not. Then Click the APPLY button.