Why can't I install Windows on the HDD from my external?

floss if it and older ide drive and not sata..there going to be a master/slave jumper..check the old drive to see if it has one. if it stata check to see what mode your mb sata ports are in ide or achi..sometime having the ports in the wrong mode wont let a drive show up. the other issue is the drive in all the way ..did you miss any of the pins connecting the drive to the mb.it the drive pusshed in all they way so that it making contact. (is the drive showing up in the bios). if it is showing up in the bios you may have to use some third party tools to do an fdisk/mbd and fdisk to make the drive fat32 or ntfs for windows to see the drive. mos times in the mbr there can be bits of the backup software left. what you did is what most theaves would do if they stoll one of those drives.there may be some software still left on the drive to stop you from copying the data from the drive.

It's definitely a SATA hard drive. Both are. Same size. Same specs. Fits like a key.

When I put the external back in it's casing and connect through the USB it shows up fine, so there can't be a connection issue. Also when I take the internal out and attach it to the USB connector, it show up fine on other computers as well.

Both are formatted as NTFS, and before attempting to install Windows I did a long format on the external (through the OS)

Oh... and that was my cheapo solution to fixing my HDD. Just wanted to switch them out to save money.

Anyway, it's not really an issue anymore b/c I bought a 750GB 7200RPM from NewEgg. I'm still curious as to why it didn't work though...