There are many option to check hard disk drive or USB flash drive (UFD) Write Speed in ubuntu, on this discuss we will use default command in ubuntu that is hdparm, hdparm is a command line utility for the Linux and Windows operating systems to set and view ATA hard disk hardware parameters. It can set parameters such as drive caches, sleep mode, power management, acoustic management, and DMA settings. Here is step to check your hard disk drive or flash drives (UFD) write speed :

Check all Mounted Device
step 1, check all mounted device in your computer using following command.
sudo fdisk -l
choose what device you would to test, hard drive or mounted flash drives (a flash drive used to recognize by system with /dev/sdb).

Check Hard Drive / Flash Drive Speed
Step 2, as I mentioned above, we use hdpram to perform this, do as following command to check your Hard Drive / Flash Drive speed. you can change /dev/sda1 with other partition in your system.
sudo hdparm -t /dev/sda1
you should get result as bellow :

 Timing buffered disk reads: 302 MB in  5.31 seconds =  91.38 MB/sec  

From result above, i found that my hard drive speed is 91.38MB/sec, a same way you can use to check your Flash Drives speed, change /dev/sda1 with your flash drive address listed in `fdisk -l`.