Dell OptiPlex 755 User Manual page 239

1 hard drive
2 drive rails (2)
3 data cable connector
4 power cable connector
7. Disconnect the data and power cables from the drive connectors.
8. Remove the four screws that secure the drive rails to the existing hard drive, and attach the drive rails to the replacement hard drive.
1 drive
2 drive rails (2)
3 screws (4)
NOTICE: Serial ATA data and power connectors are keyed for correct insertion; that is, a notch or a missing pin on one connector matches a tab or a
filled-in hole on the other connector. When connecting cables to a drive, ensure that the connectors are oriented properly before you attach the cables
to the drive.
9. Attach the data and power cables to the hard drive connectors, ensuring that the connectors are properly oriented before connecting them.
10. Slide the hard drive into the hard drive bracket.