Format USB Drive with FAT on Mac with different Allocation Unit Size

Question

I've been asked to format a USB Drive with a larger Allocation Unit Size than the default. I'm using a Mac, Snow Leopard. In Disk Utility, I am unable to see anything to do with Allocation Unit Size. A bit of Googling suggests it might be called Cluster Size, but I can't see anything for that, either. Does anyone know if it's possible to format a drive with FAT and use a larger than default Allocation Unit Size / Cluster Size?

Answer 1

For example to format FAT32 with 64kb allocation unit size:

sudo newfs_msdos -F 32 -c 128 -v VOLUMEID /dev/yourdiskdevice

with 32kb allocation unit size should be use 64 cluster size:

sudo newfs_msdos -F 32 -c 64 -v VOLUMEID /dev/yourdiskdevice

Answer 2

You should be able to do this from a command line using the newfs_msdos command. For example:

#> newfs_msdos -F 32 -V somevolumename /dev/youdiskdevice

The command takes many arguments that you may want to explore.. -c cluster-size for example.

I am sure you could google around for newfs_msdos for some usage examples and also man newfs_msdos

Hope this helps!

Answer 3

I didn't know that I had to unmount the disk in order to format the card. So I looked around to find an answer for the different errors, when all I actually needed to do was

diskutil unmount /dev/disk(the ident number)

I had to format my micro SD for a sports cam and the "cluster" size was wrong. I tried the -c 32 first, no joy. -c 64 Bingo!