This is a copy of text from http://www.6502.org , for my reference, whose source is I assume the owners/repair manual or somewhere else on the web.
Disk drive error codes (4040, 8050, 8250, sfd-1001, D9060, D9090)
The Commodore disk drives indicated hardware failures detected during boot with a blink code. Here's the meaning
from the 8050/8250 service manual:
POWER UP DIAGNOSTIC FOR 8050/8250 FLASH CODE
On power up, the three LED's on the drive should blink. The drive LED's go off and the center Tri-State LED remains
green. The system has a built-in self diagnostic which can be read in the form of a flash code. In the event of a failure,
the 3 LED's will blink in a repetitive sequence. The number of blinks between intervals will indicate the possible
location of a failure.
NUMBER OF FLASHES ERROR CAUSE COMPONENT LOCATION
1 Zero Page 6532 UC1,UE1
2 ROM $E000-$EFFF 2364 UL1
3 ROM $C000-$DFFF 2364 UH1
5 Zero Page 6530 6502 UK3 UH3
6 Common RAM $1000-$13FF 2114 UC4,UC5
7 Common RAM $2000-$23FF 2114 UD4,UD5
8 Common RAM $3000-$33FF 2114 UE4,UE5
9 Common RAM $4000-$43FF 2114 UF4,UF5
10 Controller ROM 6530 6502 UK3 UH3
If the LED's come on and stay on, the microprocessor UN1 is probably at fault.