Install Windows on External HDD:
External HDD doesn’t come with Primary Partition. They have logical Partitions. Grub looks for a primary partition to load the OS. Windows Installation doesn’t take the responsibility of creating a primary partition unlike other Linux OS installations. To create a primary partition on an External HDD follow below steps:
Go to an existing Windows installation and run the DiskPart utility. Assume that external HDD is “X”.
DiskPart > LIST DISK
SELECT DISK X
CREATE PARTITION PRIMARY
SELECT PARTITION X
FORMAT FS=NTFS QUICK
Put Windows Installation disk on the DVD Drive
CD DVD Drive/boot
Copy all the files from the DVD to drive X.
Now, Reboot your computer with only drive X attached and perform the installation.