S.C. Code § 30-7-20
The recordation of any contract in the nature of a subordination, waiver or extension of any lien on real property, created by law or by agreement of the parties, shall be upon the record of the recorded mortgage or other written instrument except that it may be recorded elsewhere, if in the book for the recording of mortgages there should be no place upon the record of the recorded mortgage or other written instrument sufficient for the recordation of such contract in the nature of a subordination, waiver or extension, in which event there shall be entered on the margin of the recorded mortgage or other written instrument in regard to which such contract in the nature of a subordination, waiver or extension has been thus recorded elsewhere an appropriate reference to such recordation, giving the names of the parties thereto and the date and the book and page where such instrument is recorded. But in any county where the records are photographed, or microphotographed, or filmed, and there shall be no place upon the record of the recorded mortgage or other written instrument or upon the margin of the recorded mortgage or other written instrument sufficient for the recordation of such contract in the nature of a subordination, waiver or extension, such documents may be separately recorded as other instruments, and notation of the place of such recordation shall be entered on the index for the mortgage or other written instrument, or in a legible manner in the jacket or other container for such photograph, microphotograph or film. Any contract in the nature of a subordination, waiver or extension of any lien on real property created by law or by agreement of the parties, to be entitled to be recorded as herein provided shall be in writing and witnessed as mortgages of real property are required to be witnessed and not probated when it is upon the original mortgage or other instrument itself, but when it is upon a separate piece then it shall be probated in the same manner as is provided by law for the probating of mortgages of real property.
S.C. Code § 30-7-20