http://downloads.dell.com/published/pages/optiplex-755.html

Set Sata operation to ATA.  There are no windows 7 drivers for this model but the vista drivers work fine with windows 7.

http://knowledge.seagate.com/articles/en_US/FAQ/218571en?language=en_US

Advanced format Drives REQUIRE INTEL RST F6 drivers. Which is any hard drive over 120 gigs after 2011.  If you set AHCI in sata operation you must use INTEL RST 10.1.0.1008.  You must extract the Files to floppy or USB flash drive and have that inserted BEFORE trying to install.

https://downloadcenter.intel.com/download/20104/

Which file to choose

Download the driver STOR_allOS_10.1.0.1008_PV.exe and one of the following F6 driver diskettes, depending on your operating system:

    STOR_F6_32_10.1.0.1008_PV.exe - for 32-bit versions of Windows 7*, Windows Vista* or Windows XP*

    STOR_F6_64_10.1.0.1008_PV.exe - for 64-bit versions of Windows 7, Windows Vista or Windows XP

I set driver to AHCI setting and used the 32b option list above.  After loading the driver, it still detected no drive and said that a mass storage controller needed to be installed.

The ATA option did not work either.