Depending on what was done and when, it appears with so many OS's installed, the boot manager has been confused. Evidently the 750Gb drive has an entry in the startup string that is in error. If the OS installed on the other drives is good, why have Windows 7 installed again on the 750Gb drive? Use the 750Gb drive for data only. In order to fix this, the OS needs to be directed to look on one of the other drives. Start by looking at the BIOS settings with the 750Gb drive attached. See which drive is selected for first boot device. If it is the 750Gb, change it to another and then try reboot. If it still persists on boot failure, then remove the Boot loader info from the 750Gb drive. Caution is advised, however. If a mistake is made in one of the other drives instead of the 750Gb, then it may not boot at all.
EasyBCD may be of some value in this. Problem is, the computer has to be running to get it installed.
http://download.cnet.com/EasyBCD/3000-2094_4-10556865.html
I think some planning and an external enclosure for the 750Gb drive may be helpful.