I've located the string StartDiskboot in both the SX280 and SX260 firmware, but so far I haven't been able to correlate the code, since I haven't found the code that refers to that string yet. But I have found one signpost string, ErrColct.c, which enabled me to locate the HardwareDefect_FW and HardwareDefectWithRestart_FW functions in both firmware. Thought you might like to see both of them:
SX260 (v101a):
ROM:FF027460 ; =============== S U B R O U T I N E =======================================
ROM:FF027460
ROM:FF027460 ; Attributes: noreturn
ROM:FF027460
ROM:FF027460 HardwareDefectWithRestart_FW ; CODE XREF: sub_FF0698DC+19Cp
ROM:FF027460 ; DATA XREF: ROM:FF02752Co
ROM:FF027460 STMFD SP!, {R4-R6,LR}
ROM:FF027464 LDR R4, =0x1DB8
ROM:FF027468 MOV R5, R0
ROM:FF02746C LDR R1, =0x1D4C0
ROM:FF027470 LDR R0, [R4,#0xC]
ROM:FF027474 MOV R3, #0x5B
ROM:FF027478 ADR R2, aErrcolct_c ; "ErrColct.c"
ROM:FF02747C BL 0x68AEC0
ROM:FF027480 LDR R0, [R4]
ROM:FF027484 CMP R0, #0
ROM:FF027488 MOV R0, #1
ROM:FF02748C STREQ R5, [R4]
ROM:FF027490 STR R0, [R4,#8]
ROM:FF027494 STR R5, [R4,#4]
ROM:FF027498 LDR R1, [R4,#0x10]
ROM:FF02749C BLX R1
ROM:FF0274A0 MOV R0, #0
ROM:FF0274A4 STR R0, [R4,#8]
ROM:FF0274A8 LDR R0, [R4,#0xC]
ROM:FF0274AC LDMFD SP!, {R4-R6,LR}
ROM:FF0274B0 B 0x68AA60
ROM:FF0274B0 ; End of function HardwareDefectWithRestart_FW
ROM:FF0274B0
ROM:FF0274B4
ROM:FF0274B4 ; =============== S U B R O U T I N E =======================================
ROM:FF0274B4
ROM:FF0274B4 ; Attributes: noreturn
ROM:FF0274B4
ROM:FF0274B4 HardwareDefect_FW ; CODE XREF: sub_FF01E810:loc_FF01E850j
ROM:FF0274B4 ; ROM:FF03B860p ...
ROM:FF0274B4 STMFD SP!, {R4-R6,LR}
ROM:FF0274B8 LDR R4, =0x1DB8
ROM:FF0274BC MOV R5, R0
ROM:FF0274C0 LDR R1, =0x1D4C0
ROM:FF0274C4 LDR R0, [R4,#0xC]
ROM:FF0274C8 MOV R3, #0x55
ROM:FF0274CC ADR R2, aErrcolct_c ; "ErrColct.c"
ROM:FF0274D0 BL 0x68AEC0
ROM:FF0274D4 LDR R0, [R4]
ROM:FF0274D8 CMP R0, #0
ROM:FF0274DC MOV R0, #1
ROM:FF0274E0 STREQ R5, [R4]
ROM:FF0274E4 STR R0, [R4,#8]
ROM:FF0274E8 STR R5, [R4,#4]
ROM:FF0274EC BL sub_FF020CF4
ROM:FF0274F0 BL sub_FF025334
ROM:FF0274F4 BL sub_FF07501C
ROM:FF0274F8 ; ---------------------------------------------------------------------------
ROM:FF0274F8 LDR R1, [R4,#0x10]
ROM:FF0274FC MOV R0, #0
ROM:FF027500 BLX R1
ROM:FF027504 MOV R0, #0
ROM:FF027508 STR R0, [R4,#8]
ROM:FF02750C LDR R0, [R4,#0xC]
ROM:FF027510 LDMFD SP!, {R4-R6,LR}
ROM:FF027514 B 0x68AA60
ROM:FF027514 ; End of function HardwareDefect_FW
ROM:FF027514
ROM:FF027518 ; ---------------------------------------------------------------------------
ROM:FF027518 STMFD SP!, {R4,LR}
ROM:FF02751C ADR R1, HardwareDefect_FW
ROM:FF027520 ADR R0, aHardwaredefect ; "HardwareDefect"
ROM:FF027524 BL loc_FF07CD40
ROM:FF027528 ; ---------------------------------------------------------------------------
ROM:FF027528 LDMFD SP!, {R4,LR}
ROM:FF02752C ADR R1, HardwareDefectWithRestart_FW
ROM:FF027530 ADR R0, aHardwaredefectwithrestar ; "HardwareDefectWithRestart"
ROM:FF027534 B loc_FF07CD40