Allocation unit size, also known as cluster size or block size, refers to the size of the chunks that a solid-state drive (SSD) or hard disk drive (HDD) is divided into. When you format a new storage device, you can pick the file system you want to use and the allocation unit size. The default allocation unit size for an NTFS drive is 4 KB. If youre only going to be storing large files on a drive, feel free to use a larger allocation unit size, as itll improve your storage efficiency. However, if you make your allocation unit size too small, it can lead to a slower system, as allocation will take longer, and if you make it too big, it will take up precious disk space.

The best allocation unit size to use depends on what youll be using the drive for. For example, if you have a partition or drive that you use exclusively for movies, it makes sense to use a large allocation unit size, as a single movie file tends to be several hundred MB or even a few GB large. So you could use the maximum allocation unit size of 2 MB, but bear in mind that this will make smaller files (like subtitle files) use that amount of space at a minimum. If you have lots of small files, then its a good idea to use a smaller allocation unit size.

If youre not sure what allocation unit size to use, you can leave it at the default value, which is selected based on the size of the partition and is designed to reduce the space that is lost and to reduce the fragmentation that occurs on the partition.