I'm formatting my WD external drive from FAT32 to NTFS. It asks for allocation unit size with options ranging from 512 bytes to 64 kilobytes.

Default is 4096 bytes.

Most files that will be on the drive are flac files ranging from 15-40 MB on average.

There will be some photos, program installation files, etc. too.

which allocation size should I choose?

Thanks.

The default unit allocation size is a good balance between space and speed, so my advice would be to stick with 4096 bytes, for NTFS.

Good, started formatting 2 minutes ago and went with the default. figured I could always re-re-format if someone suggested something better here.

You could have gone larger, but it doesn't really matter much.

The biggest effect of the allocation size is that every file will be rounded to a multiple of that. So 4k is a pretty good trade off. If you were storing millions of files less than one 1k, then the 512 byte size would make sense. If all the files you were storing were gigabytes in size, then the 64k would probably gain a little extra speed.

The default is good.

Doubtful you'd even notice a difference no matter what you chose. That said I believe in using defaults unless you have a really good reason not to.