Can you do a fresh install of Win 7 on a USB3 portable hdd
 

I have a new computer, but no hdd yet ...

but I do have a year old USB 3 seagate 1.5 TB portable hdd
that I could transfer the contents to another old hdd and wipe it clean.

Can I then plug it in to the new computer with four USB 3 plug in's and install a fresh install of win 7?

Thanks

Thus, from an external hard drive, it should not be a problem, because it's all about the OS, and it may well be synonymous with a disc.Formatting before installation.

Shouldn't be an issue. Just ensure it's always plugged in to same port. When you get your new drive use the built in Windows Easy Transfer.

Personally I'd be tempted to take the drive out the enclosure and use it internally then if you still want new drive ghost it - most manufacturers nowadays have a free program to shift stuff from one drive to another.

Edit - Don't forget to set the BIOS to boot from USB, a basic but easy potential mistake.