Which to buy

WD External Drives External Drives for PC
1

What current WD harddrive has the greatest allocation unit size?

2

The “allocation unit” is set by Windows when you format the disk, and can be changed by a re-format.

The standard for NTFS volumes is either 512-byte or 4096-byte Allocation Units.

But if you want larger allocation units (God knows why), you can just select a larger unit when you format the disk.