Denon HD2500 - "Can't Access" Message?

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Hopefully the master that is Gary is still around these parts and can shed some light on this?
I've mentioned it in Good Gig / Bad Gig and I have also searched past threads and had a look around the Denon forums and can't seem to see anything that refers to this particular message that appeared on the Denon screen on Saturday night.

On Saturday night a song was playing (there's nothing wrong with the MP3 by the way, it's not corrupt) and it just stopped. I looked at the screen and there was a "Can't Access" message on it. It was connected to USB1. It happened just the once and was fine for the rest of the evening? USB2 was fine?
Of the two external HD's (both powered and both connected to a Belkin powered hub), USB2 is the older of the two. Both are FAT32.

There are around 15,000 tracks on the HD's. On the newer one (about 4 years old actually) I have songs stashed in genre folders (Indie / Rock / Chart & Dance / Motown etc etc..). On the older one I have I have the folders as A-Z.
I've read that sometimes it's best to limit 1000 tracks per folder? I have also read that sometimes the keyboard can cause problems? Confused!
I've also seen advice given that a Defrag' could help? I am pretty much 100% certain that the timer thing that gets mentioned has been turned off on the HD's.

Not sure if this helps either but the 2500 wasn't idle for ages whilst VDJ played background music during the meal. I left it turned off.

I am going to look at the HD's in detail later on and go through them to see if there are any problems but any help or advice on what could be the cause of the message would be much appreciated.

I got away with it on this occasion but I wouldn't like it to be a regular thing.
Thanks

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Been trying to sort this out and replicate the problem.
I noticed that the older of the powered external HD's (in a 3.5" drive enclosure) was flashing. On the 12 pin power supply there is an orange light. It was flashing earlier but is glowing solid orange now - Surely this should be green if it's working properly? I've done a search for the serial number and to see what the problem is and there aren't really any answers anywhere (typical).

So, I am guessing (probably wrongly) that could there possibly be a problem caused IF this external HD with a possible dodgy power supply is connected to the same powered USB as the other one that is working fine? Could the dodgy one be the cause?

At the moment I just have the good one plugged in and I have left the Denon running and it seems ok.

[Excalibur]

Not much help, but it's perhaps useful to remember that hard drives have a finite lifespan. I try to replace mine on a rolling programme. I'm currently not using any of the ones I started out with. Not too painful, as they now seem to be cheaper, smaller, and higher storage capacity.

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Thanks, yes I agree.
I'm now going through the painful process of replacing the potentially dodgy powered one with a new one that I got a while back.
Hopefully it will do the trick.

[Solitaire Events Ltd]

I'm pretty sure this is what happened to my internal drive, but it had come loose, so I took it apart and reset it, and everything was fine.

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Cheers, I'll look at that too. I don't often use that though to be honest.

[Solitaire Events Ltd]

I meant reseated it rather than reset!

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I've yet to check out the internal HD.
I ran the Denon for 6 hours yesterday without the dodgy powered external HD and just the good one and there were no problems at all, it didn't stop once.
I've also gone through it and cleaned it up a bit.
Fingers crossed.