How can I determine the sector size on an external hard drive?

Run wmic partition get BlockSize, StartingOffset, Name, Index from a Windows Command Prompt. The output looks like:

BlockSize  Index  Name                   StartingOffset
512        0      Disk #0, Partition #0  1048576
512        1      Disk #0, Partition #1  105906176
512        0      Disk #1, Partition #0  32256

Where block size is the drive's sector size. It unfortunately doesn't list the drive letter.

Also as I understand the article, the drive will still list that it has 512 byte sectors even though internally it uses 4kb sectors. So the only way may be to get the drive's spec sheet.

I just verified this with WMIC on my windows XP box. This is the query:

C:\>wmic DISKDRIVE get bytespersector, caption  
BytesPerSector  Caption  
512             WDC WD1600AAJS-60M0A0  
4096            TrekStor HDD USB Device

The newer Toshiba external disk is reported as 4096 bytes.

I know that you dont use linux, but just for the record:

smartctl -i /dev/...

shows

Sector Size:      512 bytes logical/physical