This might also apply to other HP Scanners. 

When the light bar flashes (blinks) on your HP53xxC scanner after you power it up, this is a subtle clue to you, the user that the Power On Self Test (POST) has failed.  If this happens, you will not be able to connect the scanner to your computer via the USB interface PORT on the scanner.  In computer speak: "Your computer will not enumerate this USB peripheral".  This is because the scanner's USB port is not activated, because once the scanner's POST fails, the scanner's hardware initialization sequence has failed.  This is mentioned a little bit on this web page:

http://h10025.www1.hp.com/ewfrf/wc/document?lc=en&cc=us&docname=nps05129&product=58381&dlc=en&lang=en

Now, the hard-core among you may not be satisfied with this explanation.  Particularly after you surf the internet and discover that various folk are complaining of this problem but are reporting different and unique flash sequences.  Aha! You are probably thinking, the crippled HP scanner in its own helpless and pathetic way is trying to communicate with me, to tell me what is wrong with it.  Something akin to "Help! I've fallen and I can't get up".

Well you would be correct.  I came to the same conclusion, so I innocently logged onto the HP tech support chat line to ask a simple question: "What does 6 flashes mean?"

This was my first mistake.  The HP support guy was not programmed to answer that question.  Instead, and in spite of my repeated objections, we did the following:

1) Re-installed the driver
2) Re-booted the Scanner
3) Tried a different USB Cable
4) UPGRADED the driver from the file you can get here: http://ftp.hp.com/pub/scanners/software/sj669mu.exe
5) Tried a different computer and USB cable and installed driver and patch from scratch

Each time I pointed out to the HP tech that this was pointless, since his own webpage says the POST has failed and the USB port would not likely enumerate on a failed POST.  As I say, he was not programmed to assimilate this data.

While we were chatting, I also e-mailed HP tech support the same problem just in case there was a CHANCE to make contact with intelligent life.  Fifteen minutes later I got an e-mail back from a real sharp HP guy with all secret knowledge.  I share this here for your benefit and edification.

By the way, the POST error 6 that I had turned out to be the power supply voltage was too low.  For some reason this makes the optical scan bar raise an error flag.  I replaced the power cube, voltage went form 18V to 24V and presto, everything worked.  A lot of people seem to get error 5 or error 7 (5 flashes, pause, 5 flashes).  This means that the light bar's transportation lock is activated.  Just push the plastic switch on the side of the scanner to the unlocked position and power the scanner off for 60 seconds.

========================== Scanjet 5300C/5370C POST Diagnostic Codes ========================

(Look at the light bar on the scanner and count the number of flashes between each pause)

Number of blinks/ flashes        Failure                        Action
 
1                                   SRAM test failed               Exchange scanner
2                                   DRAM test failed               Exchange scanner
3                                   DARK calibration analog error  Exchange scanner
- if accompanied by a 2117 error, try the solutions on bps03134 before exchanging the scanner.

http://h10025.www1.hp.com/ewfrf/wc/document?lc=en&cc=us&dlc=en&product=58381&lang=en&docname=bps03134
                                               
4                                   Dark calibration digital error Exchange scanner
5                                   Home sensor failure OR        Unlock carriage and power cycle
                                    Carriage locked                   scanner
6                                   Optical error                      Exchange scanner.
7                                   Chassis lock                     
Unlock carriage and power cycle scanner - if problem persists, exchange scanner.
8                                   USB test failed                 
Remove cable and power cycle scanner, if problem persists, exchange scanner.
9                                   ADF Paper Path sensor failed     
Move the paper load lever to the number 1 position. Remove the paper, power cycle scanner, and place paper in ADF. Move the paper load lever to the number 2 position and rescan. If problem persists, exchange ADF.
10                                  ADF paper jam                     
Search for paper jam, remove paper if necessary, power cycle scanner, and rescan. If problem persists, exchange ADF.