I can't rename my flash drive.

Be sure you don't reformat for MSDOS. You want to use Mac OS Extended, Journaled. Be sure to repartition the drive first if you wish to remove NTFS.

Drive Partition and Format Pre-El Capitan

1. Open Disk Utility in the Utilities' folder.
2. After Disk Utility loads select the drive (out-dented entry with the mfgr.'s ID and size) from the side list.
3. Click on the Partition tab in Disk Utility's main window toolbar.
4. Set the number of partitions to one (1) from the Partition Scheme dropdown menu.
5. Set the Format type to Mac OS Extended (Journaled.)
6. Click on the Apply button.
7. Quit Disk Utility after the process is completed.

Drive Partition and Format for El Capitan or Sierra

1. Open Disk Utility in the Utilities' folder.
2. After Disk Utility loads select the drive (out-dented entry with the mfgr.'s ID and size) from the side list.
3. Click on the Erase tab in the Disk Utility toolbar. A panel should drop down.
4. In the drop down panel set the partition scheme to GUID. Set the Format type to Mac OS Extended (Journaled.)
5. Click on the Apply button and click on the Done button when it is activated.
6. Quit Disk Utility.

Hi,

If it's formatted in NTFS by a Windows computer, that's the problem.

Repartition and format the drive for OS X using Disk Utility.

If you have data you want to keep, don't reformat the drive until you have saved that data elsewhere (and hopefully backed it up then), as reformatting will erase anything/everything on the drive.